3
_
·
General Information 5
Construction documentation and procurement
Construction documentation
The members of the design team each produce drawings, specifications and schedules
which explain their designs to the contractor. The drawings set out in visual form how the
design is to look and how it is to be put together. The specification describes the design
requirements for the materials and workmanship, and additional schedules set out sizes
and co-ordination information not already covered in the drawings or specification. The
quantity surveyor uses all of these documents to prepare bills of quantities, which are
used to help break down the cost of the work. The drawings, specifications, schedules
and bills of quantities form the tender documentation. `Tender' is when the bills and
design information are sent out to contractors for their proposed prices and construction
programmes. `Procurement' simply means the method by which the contractor is to be
chosen and employed, and how the building contract is managed.
Traditional procurement
Once the design is complete, tender documentation is prepared and sent out to the
selected contractors (three to six depending on how large the project is) who are normally
only given a month to absorb all the information and return a price for the work.
Typically, a main contractor manages the work on site and has no labour of his own.
The main contractor gets prices for the work from subcontractors and adds profit and
preliminaries before returning the tenders to the design team. The client has the option to
choose any of the tenderers, but the selection in the UK is normally on the basis of the
lowest price. The client will be in contract with the main contractor, who in turn is in
contract with the subcontractors. The architect normally acts as the contract adminis-
trator for the client. The tender process is sometimes split to overlap part of the design
phase with a first stage tender and to achieve a quicker start on site than with a
conventional tender process.
6 Structural Engineer's Pocket Book
Construction management
Towards the end of the design process, the client employs a management contractor to
oversee the construction. The management contractor takes the tender documentation,
splits the information into packages and chooses trade contractors (a different name for a
subcontractor) to tender for the work. The main differences between construction
management and traditional procurement are that the design team can choose which
trade contractors are asked to price and the trade contractors are directly contracted to
the client. While this type of contractual arrangement can work well for straightforward
buildings it is not ideal for refurbishment or very complex jobs where it is not easy to split
the job into simple `trade packages'.
Design and Build
This procurement route is preferred by clients who want cost security and it is generally
used for projects which have economy, rather than quality of design, as the key require-
ment. There are two versions of Design and Build. This first is for the design team to work
for the client up to the tender stage, before being `novated' to work for the main
contractor. (A variant of this is a fixed sum contract where the design team remain
employed by the client, but the cost of the work is fixed.) The second method is when
the client tenders the project to a number of consortia on an outline description and
specification. A consortium is typically led by a main contractor who has employed a
design team. This typically means that the main contractor has much more control over
the construction details than with other procurement routes.
Partnering
Partnering is difficult to define, and can take many different forms, but often means
that the contractor is paid to be included as a member of the design team, where the
client has set a realistic programme and budget for the size and quality of the building
required. Partnering generally works best for teams who have worked together before,
where the team members are all selected on the basis of recommendation and past
performance. Ideally the contractor can bring his experience in co-ordinating and
programming construction operations to advise the rest of the team on choice of
materials and construction methods. Normally detailing advice can be more difficult as
main contractors tend to rely on their subcontractors for the fine detail. The actual
contractual arrangement can be as any of those previously mentioned and sometimes
the main contractor will share the risk of costs increases with the client on the basis
that they can take a share of any cost savings.
General Information 7
Drawing conventions
Drawing conventions provide a common language so that those working in the con-
struction industry can read the technical content of the drawings. It is important for
everyone to use the same drawing conventions, to ensure clear communication. Con-
struction industry drawing conventions are covered by BS EN ISO 7519 which takes over
from the withdrawn BS 1192 and BS 308.
A drawing can be put to its best use if the projections/views are carefully chosen to
show the most information with the maximum clarity. Most views in construction
drawings are drawn orthographically (drawings in two dimensions), but isometric (30
·
)
and axonometric (45
·
) projections should not be forgotten when dealing with com-
plicated details. Typically drawings are split into: location, assembly and component.
These might be contained on only one drawing for a small job. Drawing issue sheets
should log issue dates, drawing revisions and reasons for the issue.
Appropriate scales need to be picked for the different type of drawings:
Location/site plans ± Used to show site plans, site levels, roads layouts, etc. Typical
scales: 1:200, 1:500 and up to 1:2500 if the project demands.
General arrangement (GA) ± Typically plans, sections and elevations set out as ortho-
graphic projections (i.e. views on a plane surface). The practical minimum for tender or
construction drawings is usually 1:50, but 1:20 can also be used for more complicated
plans and sections.
Details ± Used to show the construction details referenced on the plans to show how
individual elements or assemblies fit together. Typical scales: 1:20, 1:10, 1:5, 1:2 or
1:1
Structural drawings should contain enough dimensional and level information to allow
detailing and construction of the structure.
For small jobs or early in the design process, `wobbly line' hand drawings can be used
to illustrate designs to the design team and the contractor. The illustrations in this
book show the type of freehand scale drawings which can be done using different line
thicknesses and without using a ruler. These sorts of sketches can be quicker to
produce and easier to understand than computer drawn information, especially in
the preliminary stages of design.
8 Structural Engineer's Pocket Book
Line thicknesses
Hatching
Steps, ramps and slopes
cut section/slab edge/element to be highlighted
elevations/infill details
demolished
structure under/hidden
gridline/centre line
outline of boundaries/adjacent parts
limit of partially viewed element/cut-backline
not at intersection
breakline straight +tube
Existing
brickwork
New
brickwork
New
blockwork
Stonework Concrete
Sawn
softwood
Hardwood Insulation Subsoil Hardcore
Mortar/
screed/
plaster
Plywood Glass Steel Damp proof course
or membrane
Stairs Ramp Landscape slope Slope/pitch
Arrow indicates ‘up’
General Information 9
Common arrangement of work sections
The Common Arrangement of Work Sections for Building Work (CAWS) is intended to
provide a standard for the production of specifications and bills of quantities for building
projects, so that the work can be divided up more easily for costing and for distribution to
subcontractors. The full document is very extensive, with sections to cover all aspects of
the building work including: the contract, structure, fittings, finishes, landscaping and
mechanical and electrical services. The following sections are extracts from CAWS to
summarize the sections most commonly used by structural engineers:
A Preliminaries/
general
conditions
A1 The project generally A2 The contract
A3 Employer's
requirements
A4 Contractor's general
costs
C Existing site/
buildings/
services
C1 Demolition C2 Alteration ± composite
items
C3 Alteration ± support C4 Repairing/renovating
concrete/masonry
C5 Repairing/renovating
metal/timber
D Groundwork D1 Investigation/
stabilization/
dewatering
D2 Excavation/filling
D3 Piling D4 Diaphragm walling
D5 Underpinning
E In situ
concrete/large
precast
concrete
E1 In situ concrete E2 Formwork
E3 Reinforcement E4 In situ concrete sundries
E5 Precast concrete
large units
E6 Composite construction
F Masonry F1 Brick/block walling F2 Stone walling
F3 Masonry accessories
G Structural/
carcassing
in metal
or timber
G1 Structural/carcassing
metal
G2 Structural/carcassing
timber
G3 Metal/timber decking
R Disposal
systems
R1 Drainage R2 Sewerage
There is a very long list of further subheadings which can be used to cover sections in
more detail (e.g. F10 is specifically for Brick/block walling). However, the list is too
extensive to be included here.
Source: CPIC (1998).
10 Structural Engineer's Pocket Book
Summary of ACE conditions of engagement
The Association of Consulting Engineers (ACE) represents the consulting sector of the
engineering profession in the UK. The ACE Conditions of Engagement, Agreement B(1),
3rd Edition (2002) is used where the engineer is appointed directly to the client and works
with an architect who is the lead consultant or contract administrator. A summary of the
Normal Services from Agreement B(1) is given below with references to the lettered work
stages (A±L) defined by the Royal Institute of British Architects (RIBA).
Feasibility
Work
Stage A
Appraisal Identification of client requirements and development
constraints by the Lead Consultant, with an initial appraisal to
allow the client to decide whether to proceed and to select
the probable procurement method.
Stage B Strategic
briefing
Confirmation of key requirements and constraints for or by
the client, including any topographical, historical or
contamination constraints on the proposals. Consider the
effect of public utilities and transport links for construction
and post construction periods on the project. Prepare a site
investigation desk study and if necessary bring the full site
investigation forward from Stage C. Identify the Project Brief,
establish design team working relationships and lines of
communication and discuss with the client any requirements
for site staff or resident engineer. Collaborate on the design
with the design team and prepare a stage report if requested
by the client or lead consultant.
Pre-construction phase
Stage C Outline
proposals
Visit the site and study any reports available regarding the site.
Advise the client on the need and extent of site investigations,
arrange quotes and proceed when quotes are approved by the
client. Advise the client of any topographical or dimensional
surveys that are required. Consult with any local or other
authorities about matters of principle and consider alternative
outline solutions for the proposed scheme. Provide advice,
sketches, reports or outline specifications to enable the Lead
Consultant to prepare his outline proposals and assist the
preparation of a Cost Plan. Prepare a report and, if required,
present to the client.
Stage D Detailed
proposals
Develop the design of the detailed proposals with the design
team for submission of the Planning Application by the Lead
Consultant. Prepare drawings, specifications, calculations and
descriptions in order to assist the preparation of a Cost Plan.
Prepare a report and, if required, present to the client.
General Information 11
Summary of ACE conditions of engagement ±
continued
Pre-construction phase ± continued
Stage E Final proposals Develop and co-ordinate all elements of the project in the overall scheme with
the design team, and prepare calculations, drawings, schedules and
specifications as required for presentation to the client. Agree a programme for
the design and construction of the Works with the client and the design team.
Stage F Production
information
Develop the design with the design team and prepare drawings, calculations,
schedules and specifications for the Tender Documentation and for Building
Regulations Approval. Prepare any further drawings and schedules necessary to
enable Contractors to carry out the Works, excluding drawings and designs for
temporary works, formwork, and shop fabrication details (reinforcement
details are not always included as part of the normal services). Produce a
Designer's Risk Assessment in line with Health & Safety CDM Regulations.
Advise the Lead Consultant on any special tender or contract conditions.
Stage G
Stage H
Tender
documents
Tender action
Assist the Lead Consultant in identifying and evaluating potential contractors
and/or specialists for the construction of the project. Assist the selection of
contractors for the tender lists, assemble Tender Documentation and issue it to
the selected tenderers. On return of tenders, advise on the relative merits of the
contractors proposals, programmes and tenders.
Construction phase
Stage J
Stage K
Mobilization
Construction to
practical completion
Assist the Client and Lead Consultant in letting the building contract,
appointing the contractor and arranging site hand over to the contractor. Issue
construction information to the contractor and provide further information to
the contractor as and when reasonably required. Comment on detailed
designs, fabrication drawings, bar bending schedules and specifications
submitted by the Contractors, for general dimensions, structural adequacy and
conformity with the design. Advise on the need for inspections or tests arising
during the construction phase and the appointment and duties of Site Staff.
Assist the Lead Consultant in examining proposals, but not including
alternative designs for the Works, submitted by the Contractor. Attend relevant
site meetings and make other periodic visits to the site as appropriate to the
stage of construction. Advise the Lead Consultant on certificates for payment
to Contractors. Check that work is being executed generally to the control
documents and with good engineering practice. Inspect the construction on
completion and, in conjunction with any Site Staff, record any defects. On
completion, deliver one copy of each of the final structural drawings to the
planning supervisor or client. Performwork or advise the Client in connection with
any claim in connection with the structural works.
Stage L After practical completion Assist the Lead Consultant with any administration of the building contract
after practical completion. Make any final inspections in order to help the Lead
Consultant settle the final account.
Source: ACE (1998).
12 Structural Engineer's Pocket Book
2
Statutory Authorities and
Permissions
Planning
Planning regulations control individuals' freedom to alter their property in an attempt to
protect the environment in UK towns, cities and countryside, in the public interest.
Different regulations and systems of control apply in the different UK regions. Planning
permission is not always required, and in such cases the planning department will issue a
Lawful Development Certificate on request and for a fee.
England and Wales
The main legislation which sets out the planning framework in England and Wales is the
Town and Country Planning Act 1990. The government's statements of planning policy
may be found in White Papers, Planning Policy Guidance Notes (PPGs), Mineral Policy
Guidance Notes (MPGs), Regional Policy Guidance Notes (RPGs), departmental circulars
and ministerial statements published by the Office of the Deputy Prime Minister (ODPM).
Scotland
The First Minister for Scotland is responsible for the planning framework. The main
planning legislation in Scotland is the Town and Country Planning Act (Scotland) 1997
and the Planning (Listed Buildings and Conservation Areas) (Scotland) Act 1997. The
legislation is supplemented by the Scottish Executive who publish National Planning
Policy Guidelines (NPPGs) which set out the Scottish policy on land use and other issues.
In addition, a series of Planning Advice Notes (PANs) give guidance on how best to deal
with matters such as local planning, rural housing design and improving small towns and
town centres.
Northern Ireland
The Planning (NI) Order 1991 could be said to be the most significant of the many
different Acts which make up the primary and subordinate planning legislation in North-
ern Ireland. As in the other UK regions, the Northern Ireland Executive publishes policy
guidelines called Planning Policy Statements (PPSs) which set out the regional policies to
be implemented by the local authority.
Building regulations and standards
Building regulations have been around since Roman times and are now used to ensure
reasonable standards of construction, health and safety, energy efficiency and access for
the disabled. Building control requirements, and their systems of control, are different for
the different UK regions.
The legislation is typically set out under a Statutory Instrument, empowered by an Act of
Parliament. In addition, the legislation is further explained by the different regions in
explanatory booklets, which also describe the minimum standards `deemed to satisfy' the
regulations. The `deemed to satisfy' solutions do not preclude designers from producing
alternative solutions provided that they can be supported by calculations and details to
satisfy the local authority who implement the regulations. Building control fees vary
around the country but are generally calculated on a scale in relation to the cost of the
work.
England and Wales
England and Wales has had building regulations since about 1189 when the first version
of a London Building Act was issued. Today the relevant legislation is the Building Act
1984 and the Statutory Instrument Building Regulations 2000. The Approved Documents
published by the ODPM are the guide to the minimum requirements of the regulations.
Applications may be made as `full plans' submissions well before work starts, or for small
elements of work as a `building notice' 48 hours before work starts. Completion certifi-
cates demonstrating Building Regulations Approval can be obtained on request. Third
parties can become approved inspectors and provide building control services.
Approved documents (as amended)
A Structure
A1 Loading
A2 Ground Movement
A3 and A4 Disproportionate Collapse
B Fire Safety
C Site Preparation and Resistance to Moisture
D Toxic Substances
E Resistance to the Passage of Sound
F Ventilation
G Hygiene
H Drainage and Waste Disposal
J Heat Producing Appliances
K Stairs, Ramps and Guards
L Conservation of Fuel and Power
M Access and Facilities for Disabled People
N Glazing ± Materials and Protection
Regulation 7 Materials and Workmanship
14 Structural Engineer's Pocket Book
Scotland
Building standards have been in existence in Scotland since around 1119 with the
establishment of the system of Royal Burghs. The three principal documents which
currently govern building control are the Building (Scotland) Act 1959 (as amended),
the Building Standards (Scotland) Regulations 1990 (as amended) and the Technical
Standards 1990 ± the explanatory guide to the regulations published by the Scottish
Executive.
Applications for all building and demolition work must be made to the local authority,
who assess the proposals for compliance with the technical standards, before issuing a
building warrant, which is valid for five years. Unlike the other regions in the UK, work
may only start on site once a warrant has been obtained. Buildings may only be occupied
at the end of the construction period once the local authority have issued a completion
certificate. Building control departments typically will only assess very simple structural
proposals and for more complicated work, qualified engineers must `self-certify' their
proposals.
Technical standards
A General and Definitions
B Fitness of Materials
C Structure
D Structural Fire Precautions
E Means of Escape from Fire
F Heat Producing Installations and Storage of Liquid and Gaseous Fuels
G Preparation of Sites and Resistance to Moisture
H Resistance to Transmission of Sound
J Conservation of Fuel and Power
K Ventilation of Buildings
M Drainage and Sanitary Facilities
N Electrical Installations
P Miscellaneous Hazards
Q Facilities for Dwellings
R Solid Waste Storage, Dungsteads and Farm Effluent Tanks
S Stairs, Ramps and Protective Barriers
Statutory Authorities and Permissions 15
Northern Ireland
The main legislation, policy and guidelines in Northern Ireland are the Building Regula-
tions (Northern Ireland) Order 1979 as amended by the Planning and Building Regula-
tions (Northern Ireland) (Amendment) Order 1990; the Building Regulations (NI) 2000
and the technical booklets ± which describe the minimum requirements of the regulations
published by the Northern Ireland Executive.
Building regulations in Northern Ireland are the responsibility of the Department of
Finance and Personnel and are implemented by the district councils. Until recently the
regulations operated on strict prescriptive laws, but the system is now very similar to the
system in England and Wales. Applicants must demonstrate compliance with the
`deemed to satisfy' requirements. Applications may be made as a `full plans' submission
well before work starts, or as a `building notice' for domestic houses just before work
starts. Builders must issue stage notices for local authority site inspections. Copies of the
stage notices should be kept with the certificate of completion by the building owner.
Technical booklets
A Interpretation and General
B Materials and Workmanship
C Preparation of Sites and Resistance to Moisture
D Structure
E Fire Safety
F Conservation of Fuel and Power
G Sound Insulation
H Stairs, Ramps and Guarding
J Solid Waste in Buildings
K Ventilation
L Heat Producing Appliances and LPG Systems
N Drainage
P Sanitary Appliances and Unvented Hot Water Storage Systems
R Access and Facilities for Disabled People
V Glazing
16 Structural Engineer's Pocket Book
Listed buildings
In the UK, buildings of `special architectural or historic interest' can be listed to ensure
that their features are considered before any alterations are agreed to the exterior or
interior. Buildings may be listed because of their association with an important architect,
person or event or because they are a good example of design, building type, construc-
tion or use of material. Listed building consent must be obtained from the local authority
before any work is carried out on a listed building. In addition, there may be special
conditions attached to ecclesiastical, or old ecclesiastical, buildings or land by the local
diocese or the Home Office.
England and Wales
English Heritage (EH) in England and CADW in Wales work for the government to identify
buildings of `special architectural or historic interest'. All buildings built before 1700 (and
most buildings between 1700 and 1840) with a significant number of original features
will be listed. A building normally must be over 30 years old to be eligible for listing. There
are three grades: I, II* and II, and there are approximately 500000 buildings listed in
England, with about 13 000 in Wales. Grades I and II* are eligible for grants from EH for
urgent major repairs and residential listed buildings may be VAT zero rated for approved
alterations.
Scotland
Historic Scotland maintains the lists and schedules for the Scottish Executive. All buildings
before 1840 of substantially unimpaired character can be listed. There are over 40 000
listed buildings divided into three grades: A, B and C. Grade A is used for buildings of
national or international importance or little altered examples of a particular period, style
or building type, while a Grade C building would be of local importance or be a
significantly altered example of a particular period, style or building type.
Northern Ireland
The Environment and Heritage Service (EHS) within the Northern Ireland Executive has
carried out a survey of all the building stock in the region and keeps the Northern Ireland
Buildings Database. Buildings must be at least 30 years old to be listed and there are
currently about 8500 listed buildings. There are three grades of listing: A, B+ and B (with
two further classifications B1 and B2) which have similar qualifications to the other UK
regions.
Statutory Authorities and Permissions 17
Conservation areas
Local authorities have a duty to designate conservation areas in any area of `special
architectural or historic interest' where the character or appearance of the area is worth
preserving or enhancing. There are around 8500 conservation areas in England and
Wales, 600 in Scotland and 30 in Northern Ireland. The character of an area does not
just come from buildings and so the road and path layouts, greens and trees, paving and
building materials and public and private spaces are protected. Conservation area consent
is required from the local authority before work starts to ensure any alterations do not
detract from the area's appearance.
Tree preservation orders
Local authorities have specific powers to protect trees by making Tree Protection Orders
(TPOs). Special provisions also apply to trees in conservation areas. A TPO makes it an
offence to cut down, lop, top, uproot, wilfully damage or destroy the protected tree
without the local planning authority's permission. All of the UK regions operate similar
guidelines with slightly different notice periods and penalties.
The owner remains responsible for the tree(s), their condition and any damage they may
cause, but only the planning authority can give permission to work on them. Arboricul-
turalists (who can give advice on work which needs to be carried out on trees) and
contractors (who are qualified to work on trees) should be registered with the Arbor-
icultural Association. In some cases (including if the tree is dangerous) no permission is
required, but notice (about 5 days (or 6 weeks in a conservation area) depending on the
UK region) must be given to the planning authority. When it is agreed that a tree can be
removed, this is normally on the condition that a similar tree is planted as a replacement.
Permission is generally not required to cut down or work on trees with a trunk less than
75 mm diameter (measured at 1.5 m above ground level) or 100mm diameter if thinning
to help the growth of other trees. Fines of up to £20 000 can be levied if work is carried
out without permission.
18 Structural Engineer's Pocket Book
Archaeology and ancient monuments
Archaeology in Scotland, England and Wales is protected by the Ancient Monuments and
Archaeology Areas Act 1979, while the Historic Monuments and Archaeology Objects
(NI) Order 1995 applies in Northern Ireland.
Archaeology in the UK can represent every period from the camps of hunter gatherers
10 000 years ago to the remains of twentieth century industrial and military activities.
Sites include places of worship, settlements, defences, burial grounds, farms, fields and
sites of industry. Archaeology in rural areas tends to be very close to the ground surface,
but in urban areas, deep layers of deposits were built up as buildings were demolished
and new buildings were put directly on the debris. These deposits, often called `medieval
fill', are an average of 5 m deep in places like the City of London and York.
Historic or ancient monuments are structures which are of national importance. Typically
monuments are in private ownership but are not occupied buildings. Scheduled monu-
ment consent is required for alterations and investigations from the regional heritage
bodies: English Heritage, Historic Scotland, CADW in Wales and EHS in Northern Ireland.
Each of the UK regions operates very similar guidelines in relation to archaeology, but
through different frameworks and legislation. The regional heritage bodies develop the
policies which are implemented by the local authorities. These policies are set out in PPG
16 for England and Wales, NPPG 18 for Scotland and PPS 6 for Northern Ireland. These
guidance notes are intended to ensure that:
1. Archaeology is a material consideration for a developer seeking planning permission.
2. Archaeology strategy is included in the urban development plan by the local planning
authority.
3. Archaeology is preserved, where possible, in situ.
4. The developer pays for the archaeological investigations, excavations and reporting.
5. The process of assessment, evaluation and mitigation is a requirement of planning
permission.
6. The roles of the different types of archaeologists in the processes of assessment,
evaluation and mitigation are clearly defined.
Statutory Authorities and Permissions 19
Where `areas of archaeological interest' have been identified by the local authorities, the
regional heritage bodies act as curators (English Heritage, Historic Scotland, CADW in
Wales and EHS in Northern Ireland). Any developments within an area of archaeological
interest will have archaeological conditions attached to the planning permission to ensure
that the following process is put into action:
1. Early consultation between the developers and curators so that the impact of the
development on the archaeology (or vice versa) can be discussed and the developer
can get an idea of the restrictions which might be applied to the site, the construction
process and the development itself.
2. Desk study of the site by an archaeologist.
3. Field evaluation by archaeologists using field walking, trial pits, boreholes and/or
geophysical prospecting to support the desk study.
4. Negotiation between the site curators and the developer's design team to agree the
extent of archaeological mitigation. The developer must submit plans for approval by
the curators.
5. Mitigation ± either preservation of archaeology in situ or excavation of areas to be
disturbed by development. The archaeologists may have either a watching brief over
the excavations carried out by the developer (where they monitor construction work
for finds) or on significant sites, carry out their own excavations.
6. Post-excavation work to catalogue and report on the archaeology, either store or
display the findings.
Generally the preliminary and field studies are carried out by private consultants and
contractors employed by the developers to advise the local authority planning depart-
ment. In some areas advice can also be obtained from a regional archaeologist. In
Northern Ireland, special licences are required for every excavation which must be under-
taken by a qualified archaeologist. In Scotland, England and Wales, the archaeological
contractors or consultants have a `watching brief'.
Field evaluations can often be carried out using geotechnical trial pits with the excav-
ations being done by the contractor or the archaeologist depending on the importance of
the site. If an interesting find is made in a geotechnical trial pit and the archaeologists
would like to keep the pit open for inspection by, say, the curators, the developer does
not have to comply if there would be inconvenience to the developer or building users, or
for health and safety reasons.
Engineers should ensure for the field excavation and mitigation stages that the archae-
ologists record all the features in the excavations up to this century's interventions as
these records can be very useful to the design team. Positions of old concrete footings
could have as much of an impact on proposed foundation positions as archaeological
features!
20 Structural Engineer's Pocket Book
Party Wall etc. Act
The Party Wall etc. Act 1996 came into force in 1997 throughout England and Wales. In
2002 there is no equivalent legislation in Northern Ireland and in Scotland, the Law of the
Tenement is only in draft form.
Different sections of the Party Wall Act apply, depending on whether you propose to carry
out work to an existing wall or structure shared with another property; build a free-
standing wall or the wall of a building astride a boundary with a neighbouring property,
and/or excavate within 3 m of a neighbouring building or structure. Work can fall within
several sections of the Act at one time. A building Owner must notify his neighbours and
agree the terms of a Party Wall Award before starting any work.
The Act refers to two different types of Party Structure: `Party Wall' and `Party Fence Wall'.
Party Walls are loosely defined as a wall on, astride or adjacent to a boundary enclosed by
building on one or both sides. Party Fence Walls are walls astride a boundary but not part
of a building; it does not include things like timber fences. A Party Structure is a wide term
which can sometimes include floors or partitions.
The Notice periods and sections 1, 2 and 6 of the Act are most commonly used, and are
described below.
Notice periods and conditions
In order to exercise rights over the Party Structures, the Act says that the Owner must give
Notice to Adjoining Owners; the building Owner must not cause unnecessary inconveni-
ence, must provide compensation for any damage and must provide temporary protec-
tion for buildings and property where necessary. The Owner and the Adjoining Owner in
the Act are defined as anyone with an interest greater than a tenancy from year to year.
Therefore this can include shorthold tenants, long leaseholders and freeholders for any
one property.
A building Owner, or surveyor acting on his behalf, must send a Notice in advance of the
start of the work. Different Notice periods apply to different sections of the Act, but work
can start within the Notice period with the written agreement of the Adjoining Owner. A
Notice is only valid for one year from the date that it is served and must include the
Owner's name and address, the building's address (if different); a clear statement that the
Notice is under the provisions of the Act (stating the relevant sections); full details of the
proposed work (including plans where appropriate) and the proposed start date for the
work.
The Notice can be served by post, in person or fixed to the adjoining property in a
`conspicuous part of the premises'. Once the Notice has been served, the Adjoining
Owner can consent in writing to the work or issue a counter Notice setting out any
additional work he would like to carry out. The Owner must respond to a counter Notice
within 14 days. If the Owner has approached the Adjoining Owners and discussed the
work with them, the terms of a Party Wall Award may have already been agreed in writing
before a Notice is served.
Statutory Authorities and Permissions 21
If a Notice is served and the Adjoining Owner does not respond within 14 days, a dispute
is said to have arisen. If the Adjoining Owner refuses to discuss terms or appoint a surveyor
to act on his behalf, the Owner can appoint a surveyor to act on behalf of the Adjoining
Owner. If the Owners discuss, but cannot agree terms they can jointly appoint a surveyor
(or they can each appoint one) to draw up the Party Wall Award. If two surveyors cannot
agree, a nominated Third Surveyor can be called to act impartially. In complex cases, this
can often take over a year to resolve and in such cases the Notice period can run out,
meaning that the process must begin again by serving another Notice. In all cases, the
surveyors are appointed to consider the rights of the Owner over the wall and not to act
as advocates in the negotiation of compensation! The building Owner covers the costs
associated with all of the surveyors and experts asked about the work.
When the terms have been agreed, the Party Wall Award should include a description (in
drawings and/or writing) of what, when and how work is to be carried out; a record of
the condition of the adjoining Owner's property before work starts; arrangements to
allow access for surveyors to inspect while the works are going on and say who will pay
for the cost of the works (if repairs are to be carried out as a shared cost or if the adjoining
Owner has served a counter Notice and is to pay for those works). Either Owner has 14
days to appeal to the County Court against an Award if an Owner believes that the person
who has drafted the Award has acted beyond their powers.
An Adjoining Owner can ask the owner for a `bond'. The bond money becomes the
property of the Adjoining Owner (until the work has been completed in accordance with
the Award) to ensure that funds are available to pay for the completion of the works in
case the Owner does not complete the works.
The Owner must give 14 days' Notice if his representatives are to access the Adjoining
Owner's property to carry out or inspect the works. It is an offence to refuse entry or
obstruct someone who is entitled to enter the premises under the Act if the offender
knows that the person is entitled to be there. If the adjoining property is empty, the
Owner's workmen and own surveyor or architect may enter the premises if they are
accompanied by a police officer.
22 Structural Engineer's Pocket Book
Section 1 ± new building on a boundary line
Notice must be served to build on or astride a boundary line, but there is no right to build
astride if your neighbour objects. You can build foundations on the neighbouring land if
the wall line is immediately adjacent to the boundary, subject to supervision. The Notice is
required at least 1 month before the proposed start date.
Section 2 ± work on existing party walls
The most commonly used rights over existing Party Walls include cutting into the wall to
insert a DPC or support a new beam bearing; raising, underpinning, demolishing and/or
rebuilding the Party Wall and/or providing protection by putting a flashing from the
higher over the lower wall. Minor works such as fixing shelving, fitting electrical sockets or
replastering are considered to be too trivial to be covered in the Act.
A building Owner, or Party Wall Surveyor acting on the Owner's behalf must send a
Notice at least 2 months in advance of the start of the work.
Section 6 ± excavation near neighbouring buildings
Notice must be served at least 1 month before an Owner intends to excavate or construct
a foundation for a new building or structure within 3 m of an adjoining Owner's building
where that work will go deeper than the adjacent Owner's foundations, or within 6 m of
an adjoining Owner's building where that work will cut a line projecting out at 45
·
from
the bottom of that building's foundations. This can affect neighbours who are not
immediately adjacent. The Notice must state whether the Owner plans to strengthen or
safeguard the foundations of the Adjoining Owner. Adjoining Owners must agree
specifically in writing to the use of `special foundations' ± these include reinforced
concrete foundations. After work has been completed, the Adjoining Owner may request
particulars of the work, including plans and sections.
Source: DETR (1997).
Statutory Authorities and Permissions 23
CDM
The Construction Design & Management (CDM) Regulations 1994 were developed to
assign responsibilities for health and safety to the client, design team and principal con-
tractor. The Approved Code of Practice is published by the Health and Safety Executive for
guidance to the Regulations.
The client is required to appoint a planning supervisor (PS) who has overall responsibility
for co-ordinating health and safety aspects of the design and planning stages of a project.
The duties of the PS can theoretically be carried out by any of the traditional design team
professionals. The PS must ensure that the designers avoid, minimize or control health
and safety risks for the construction and maintenance of the project, as well as ensuring
that the contractor is competent to carry out the work.
The PS prepares the pre-contract health and safety plan for inclusion in the tender
documents which should include project-relevant health and safety information gathered
from the client and designers. This should highlight any unusual aspects of the project
(also highlighted on the drawings) that a competent contractor would not be expected to
know. This document is taken on by the successful principal contractor and developed
into the construction phase health and safety plan by the addition of the contractor's
health and safety policy, risk assessments and method statements as requested by the
designers. The health and safety plan is intended to provide a focus for the management
and prevention of health and safety risks as the construction proceeds.
The health and safety file is generally compiled at the end of the project by the contractor
and the PS who collect the design information relevant to the life of the building. The PS
must ensure that the file is compiled and passed to the client or the people who will use,
operate, maintain and/or demolish the project. A good health and safety file will be a
relatively compact maintenance manual including information to alert those who will be
owners, or operators of the new structure, to the risks which must be managed when the
structure and associated plant is maintained, repaired, renovated or demolished. After
handover the client is responsible for keeping the file up to date.
24 Structural Engineer's Pocket Book
3
Design Data
Design data checklist
The following design data checklist is a useful reminder of all of the limiting criteria which
should be considered when selecting an appropriate structural form:
.
Description/building use
.
Client brief and requirements
.
Site constraints
.
Loadings
.
Structural form: load transfer, stability and robustness
.
Materials
.
Movement joints
.
Durability
.
Fire resistance
.
Performance criteria: deflection, vibration, etc.
.
Temporary works and construction issues
.
Soil conditions, foundations and ground slab
.
Miscellaneous issues
Structural form, stability and robustness
Structural form
It is worth trying to remember the different structural forms when developing a scheme
design. A particular structural form might fit the vision for the form of the building. Force
or moment diagrams might suggest a building shape. The following diagrams of struc-
tural form are intended as useful reminders:
Couple Tied rafter King post Queen post
Howe
(>10 m steel/
timber)
Double howe
(8–15 m steel/
timber)
Fink
(>10 m steel/
timber)
Bowshing
(20–40 m steel)
Umbrella
(~13 m steel)
Saw tooth
(~5 m steel)
Bowshing Thrust
Scissor
(6–10 m steel/
timber)
Double fink
(5–14 m timber)
(8–13 m steel)
Northlight
(>5 m steel)
Northlight
(5–15 m steel)
Fan
(8–15 m steel)
French truss
(12–20 m steel)
Pratt Warren Modified warren
Howe Fink Modified fink
Double lattice Vierendeel
TRUSSES
GIRDERS
Double scissor
(10–13 m steel/
timber)
26 Structural Engineer’s Pocket Book
PORTAL FRAMES
All fixed 2 pin 2 pin mansard 3 pin
Thrust Tied 3 pin
ARCHES
SUSPENSION
Cable stay Suspension Closed suspension
Solid Piers Chevron Diaphragm
Ply/ply
stressed skin Ply web
Ply/timber
stressed skin Flitched
Gravity or
reinforced earth
WALLS
TIMBER
RETAINING WALLS
Embedded Cantilever
Design Data 27
Stability
Stability of a structure must be achieved in two orthogonal directions. Circular structures
should also be checked for rotational failure. The positions of movement and/or acoustic
joints should be considered and each part of the structure should be designed to be
independently stable and robust. Lateral loads can be transferred across the structure
and/or down to the foundations by using any of the following methods:
.
Cross bracing which carries the lateral forces as axial load in diagonal members.
.
Diaphragm action of floors or walls which carry the forces by panel/plate/shear action.
.
Frame action with ‘fixed’ connections between members and ‘pinned’ connections at
the supports.
.
Vertical cantilever columns with ‘fixed’ connections at the foundations.
.
Buttressing with diaphragm, chevron or fin walls.
Stability members must be located on the plan so that their shear centre is aligned with
the resultant of the overturning forces. If an eccentricity cannot be avoided, the stability
members should be designed to resist the resulting torsion across the plan.
Robustness and disproportionate collapse
All structural elements should be effectively tied together in each of the two orthogonal
directions, both horizontally and vertically. This is generally achieved by specifying con-
nections in steel buildings as being of certain minimum size, by ensuring that reinforced
concrete junctions contain a minimum area of steel bars and by using steel straps to
connect walls and floors in masonry structures. It is important to consider robustness
requirements early in the design process.
The Building Regulations require buildings of five or more storeys (excluding the roof) to
be designed for disproportionate collapse. This is intended to ensure that accidental
damage to elements of the building structure cannot cause the collapse of a dispropor-
tionately large area of a building. The disproportionate collapse requirement for public
buildings with a roof span of more than 9 m appears to have been removed from the
regulations.
Typically the Building Regulations require that any collapse caused by the failure of a
single structural element should be limited to an area of 70m
2
or 15% of any storey area
(whichever is the lesser). Alternatively the designer can strengthen the structure to with-
stand the ‘failure’ of certain structural supports in order to prevent disproportionate
collapse. In some circumstances the structure cannot be arranged to avoid the occurrence
of ‘key elements’, which support disproportionately large areas of the building. These ‘key
elements’ must be designed as protected members (to the code of practice for the
relevant structural material) to provide extra robustness and damage resistance.
28 Structural Engineer’s Pocket Book
Structural movement joints
Joints should be provided to control temperature, moisture, acoustic and ground move-
ments. Movement joints can be difficult to waterproof and detail and therefore should be
kept to a minimum. The positions of movement joints should be considered for their
effect on the overall stability of the structure.
Primary movement joints
Primary movement joints are required to prevent cracking where buildings (or parts of
buildings) are large, where a building spans different ground conditions, changes height
considerably or where the shape suggests a point of natural weakness. Without detailed
calculation, joints should be detailed to permit 15–25 mm movement. Advice on joint
spacing for different building types can be variable and conflicting. The following figures
are some approximate guidelines based on the building type:
Concrete 25 m (e.g. for roofs with large thermal differentials)–
50m c /c.
Steel industrial buildings 100m typical–150m maximum c /c.
Steel commercial buildings 50 m typical–100 m maximum c /c.
Masonry 40 m–50 m c /c.
Secondary movement joints
Secondary movement joints are used to divide structural elements into smaller elements
to deal with the local effects of temperature and moisture content. Typical joint spacings
are:
Clay bricks Up to 12 mc/c on plan (6m from corners) and 9 m
vertically or every three storeys if the building is greater
than 12 m or four storeys tall.
Concrete blocks 3 m–7m c/c.
Hardstanding 70 m c/c.
Steel roof sheeting 20 mc/c down the slope, no limit along the slope.
Design Data 29
Fire resistance periods for structural elements
Fire resistance of structure is required to maintain structural integrity to allow time for the
building to be evacuated. Generally, roofs do not require protection. Architects typically
specify fire protection in consultation with the engineer.
Building types Minimum period of fire resistance
minutes
Basement
storey
including
floor over
Ground or upper storey
Depth of a
lowest
basement
Height of top floor above
ground, in a building or
separated part of a building
>10m <10m >5m <18m <30m <120m
Residential flats and
maisonettes
90 60 30
1
60
2
90
2
120
2
Residential houses n/a 30
1
30
1
60
3
n/a n/a
Institutional residential
4
90 60 30
1
60 90 120
5
Office not sprinklered 90 60 30
1
60 90 X
sprinklered 60 60 30
1
30
1
60 120
5
Shops &
commercial
not sprinklered 90 60 60 60 90 X
sprinklered 60 60 30
1
60 60 120
5
Assembly &
recreation
not sprinklered 90 60 60 60 90 X
sprinklered 60 60 30
1
60 60 120
5
Industrial not sprinklered 120 90 60 90 120 X
sprinklered 90 60 30
1
60 90 120
5
Storage and
other non-
residential
not sprinklered 120 90 60 90 120 X
sprinklered 90 60 30
1
60 90 120
5
Car park for
light vehicles
open sided n/a n/a 15
1
15
1
15
1
60
all others 90 60 30
1
60 90 120
5
NOTES:
X Not permitted
1. Increased to 60 minutes for compartment walls with other fire compartments or 30 minutes
for elements protecting a means of escape.
2. Reduced to 30 minutes for a floor in a maisonette not contributing to the support of the
building.
3. To be 30 minutes in the case of three storey houses and 60 minutes for compartment walls
separating buildings.
4. NHS hospitals should have a minimum of 60 minutes.
5. Reduced to 90 minutes for non-structural elements.
6. Should comply with Building Regulations: B3 section 12.
Source: Building Regulations Approved Document B (1991).
30 Structural Engineer’s Pocket Book
Typical building tolerances
SPACE BETWEEN WALLS
SPACE BETWEEN COLUMNS
Brickwork ± 20 mm
Blockwork ± 21
Timber ± 32
Steel ± 12 mm
Timber ± 12
Brickwork 10 mm
Blockwork 10
In situ concrete 17
Precast concrete 11
Steel 6 mm
Timber 10
In situ concrete 12
Precast concrete 10
WALL VERTICALITY
COLUMN VERTICALITY
Maximum Maximum
VERTICAL POSITION OF BEAMS VERTICAL POSITION OF FLOORS
Steel ± 20 mm
Timber ± 20
In situ concrete ± 22
Precast concrete ± 23
In situ concrete ± 15 mm
Precast concrete ± 15
PLAN POSITION
FLATNESS OF FLOORS
In situ concrete ± 24
Precast concrete ± 18
In situ concrete ± 18
Precast concrete ± 13
In situ concrete 5 mm
Floor screed 5
3 m straight edge
max
Brickwork ± 10 mm
Steel ± 10
Timber ± 10
In situ concrete ± 12
Precast concrete ± 10
Source: BS 5606: 1990.
Design Data 31
Historical use of building materials
Masonry and timber
Non hydraulic
lime mortar
G
e
o
r
g
i
a
n
i
n
c
l
u
d
i
n
g
W
i
l
l
i
a
m

P
EI
_
M
max
=
wEI
P
sec
aL
2
_ _
÷1
_ _
Maximum compressive stress s
c max
=
My
I
÷
P
A
Maximum deflection d
max
=
÷M
P
÷
wL
2
8P
WkN/m
80 Structural Engineer's Pocket Book
Rigid frames under lateral loads
Rigid or plane frames are generally statically indeterminate. A simplified method of
analysis can be used to estimate the effects of lateral load on a rigid frame based on its
deflected shape, and assumptions about the load, shared between the columns. The
method assumes notional pinned joints at expected points of contraflexure, so that the
equilibrium system of forces can be established by statics. The vertical frame reactions as a
result of the lateral loads are calculated by taking moments about the centre of the frame.
The following methods deal with lateral loads on frames, but similar assumptions can be
made for vertical analysis (such as treating beams as simply supported) so that horizontal
and vertical moments and forces can be superimposed for use in the sizing and design of
members.
Rigid frame with infinitely stiff beam
It is assumed that the stiffness of the top beam will spread the lateral load evenly between
the columns. From the expected deflected shape, it can be reasonably assumed that each
column will carry the same load. Once the column reactions have been assumed, the
moments at the head of the columns can be calculated by multiplying the column height
by its horizontal base reaction. As the beam is assumed to be infinitely stiff, it is assumed
that the columns do not transfer any moment into the beam.
F
I = ∞
F
3
F
3
F
3
F
3
Fh
3
h
B.M.D
Basic and Shortcut Tools for Structural Analysis 81
Rigid frame with beams and columns of constant stiffness (EI)
As the top beam is not considerably stiffer than the columns, it will tend to flex and cause
a point of contraflexure at mid span, putting extra load on the internal columns. It can be
assumed that the internal columns will take twice the load (and therefore moment) of the
external columns. As before, the moments at the head of columns can be calculated by
multiplying the column height by its horizontal base reaction. The maximum moment in
the beam due to horizontal loading of the frame is assumed to equal the moment at the
head of the external columns.
Multi-storey frame with beams and columns
of constant stiffness
For a multi-storey frame, points of contraflexure can be assumed at mid points on beams
and columns. Each storey is considered in turn as a separate subframe between the
column points of contraflexure. The lateral shears are applied to the subframe columns in
the same distribution as the single storey frames, so that internal columns carry twice the
load of the external columns. As analysis progresses down the building, the total lateral
shear applied to the top of each subframe should be the sum of the lateral loads applied
above the notional point of contraflexure. The shears are combined with the lateral load
applied to the subframe, to calculate lateral shear reactions at the bottom of each
subframe. The frame moments in the columns due to the applied lateral loads increase
towards the bottom of the frame. The maximum moments in the beam due to lateral
loading of the frame are assumed to equal the difference between the moments at the
external columns.
n=number of columns
F
2(n – 1)
F
(n – 1)
F
2(n –1)
Fh
4
Fh
4
Fh
4
Fh
4
Fh
4
Fh
2
Fh
4
h
b
b/2
Assumed point of
contraflexure
F
B.M.D
82 Structural Engineer's Pocket Book
b/2
h
h
h
h
2
F
2
F
2
F
1
F
3
b
PART FRAME EXAMPLE
FULL FRAME EXAMPLE
b
F
1
B.M.D
B.M.D
F
2
F
1
h
F
1
h
F
1
h
M
b
M
b
=
M
b M
b
M
b
F
3
ΣF
4
ΣF
4
ΣF
4
ΣF
4
ΣF
2
ΣF
2
F
1
+F
2
2
F
1
+F
2
4
F
1
+F
2
4
F
1
+F
2
4
(F
1
+F
2
)h
F
1
+F
2
2
F
1
+F
2
2
F
1
+F
2
2
F
1
+F
2
2
F
1
+F
2
4
F
1
+F
2
+F
3
2
F
1
+F
2
+F
3
2
F
1
+F
2
+F
3
2
8
(2F
1
+F
2
)h
8
(F
1
+F
2
)h
4
(F
1
+F
2
)h
8
8
8 4
F
1
4
F
1
2
F
1
2
F
1
4
F
1
4
F
1
2
F
1
4
F
1
4
F
1
4
where
Multi storey frame ± continued
8
3
Plates
Johansen's yield line theory studies the ultimate capacity of plates. Deflection needs to be
considered in a separate elastic analysis. Yield line analysis is a powerful tool which should
not be applied without background reading and a sound understanding of the theory.
The designer must try to predict a series of failure crack patterns for yield line analysis by
numerical or virtual work methods. Crack patterns relate to the expected deflected shape
of the slab at collapse. For any one slab problem there may be many potential modes of
collapse which are geometrically and statically possible. All of these patterns should be
investigated separately. It is possible for the designer to inadvertently omit the worst case
pattern for analysis which could mean that the resulting slab might be designed with
insufficient strength. Crack patterns can cover whole slabs, wide areas of slabs or local
areas, such as failure at column positions or concentrated loads. Yield line moments are
typically calculated as kNm/m width of slab.
The theory is most easily applied to isotropic plates which have the same material
properties in both directions. An isotropic concrete slab is of constant thickness and
has the same reinforcement in both directions. The reinforcement should be detailed to
suit the assumptions of yield line analysis. Anisotropic slabs can be analysed if the `degree
of anisotropy' is selected before a standard analysis. As in the analysis of laterally loaded
masonry panels, the results of the analysis can be transformed on completion to allow for
the anisotropy.
The simplest case to consider is the isotropic rectangular slab:
Simple
support
Fixed
support
Column
support
b
a
i =3 i =4
i =1
i =2
84 Structural Engineer's Pocket Book
The designer must decide on the amount of fixity, i, at each support position. Generally
i =0 for simple support and i =1 for fixed or encastre supports. The amount of fixity
determines how much moment is distributed to the top of the slab m', where m' =im
and m is the moment in the bottom of the slab.
Fixed supports reduce the sagging moments, m, in the bottom of the slab. The distance
between the points of zero moment can be considered as a `reduced effective length', L
r
.
L
r
=
2L

(1 ÷i
4
)
_ _ ¸
So that the design moment is: M =
wa
r
b
r
8 1 ÷
a
r
b
r
÷
b
r
a
r
_ _
For fixity on all sides of a square slab (where a =b=L) the design moment,
M = wL
2
,24 kNm,m.
For a point load or column support, M = P,2¬ kNm/m.
L
r
m′ =m
4
m′ =m
3
M
Basic and Shortcut Tools for Structural Analysis 85
Selected yield line solutions
These patterns are some examples of those which need to be considered for a given slab.
Yield line analysis must be done on many different crack patterns to try to establish the
worst case failure moment. Both top and bottom steel should be considered by examin-
ing different failure patterns with sagging and hogging crack patterns.
a
b
m
i
2
i
4
i
3
i
1
a
r
=
2a

1 ÷i
1
_ m =
wa
r
b
r
3
2
÷3
a
b
r
÷i
2
1 ÷2
b
r
a
_ _
For opposite case, exchange a and b, I
1
and I
2
.
i
2
i
1
b d
c
a
m
F = 0.6
(a ÷c)i
1
÷(b ÷d)i
2
a ÷b ÷c ÷d
m
0
=
3wab
8 2 ÷
a
b
÷
b
a
_ _
m =
m
0
÷0.15wcd
1 ÷F
a4b42a
m′
d
c m
/
=
w
6
(c
2
÷d
2
)
Bottom steel required for main span.
m
m′
c c a
if c = 0.35a. m = m
/
=
wa
2
16
86 Structural Engineer's Pocket Book
m
m ′
h
c
m =
wh
2
3
0.39 ÷
c
h
_ _
. m
/
=
wc
2
6
If c = 0.33h. m = m
/
=
wa
2
55
m
m =
wh
2
2
0.33 ÷
c
2h
_ _2
3
_ _
Basic and Shortcut Tools for Structural Analysis 87
Torsion
Elastic torsion of circular sections:
T
J
=
t
r
=
Gf
L
Where, T is the applied torque, J is the polar moment of inertia, t is the torsional shear
stress, r is the radius, f is the angle of twist, G is the shear modulus of elasticity and L is
the length of member.
The shear strain, g, is constant over the length of the member and rf gives the displace-
ment of any point along the member. Materials yield under torsion in a similar way to
bending. The material has a stress/strain curve with gradient G up to a limiting shear
stress, beyond which the gradient is zero.
The torsional stiffness of a member relies on the ability of the shear stresses to flow in a
loop within the section shape which will greatly affect the polar moment of area, which is
calculated from the relationship J =
_
r
2
dA. This can be simplified in some closed loop
cases to J = I
zz
= I
xx
÷I
yy
.
Therefore for a solid circular section, J = pd
4
,32 for a solid square bar, J = 5d
4
,36 and
for thin walled circular tubes, J = p(d
4
outer
÷r
4
inner
),32 or J = 2pr
3
t and the shear stress,
t = T,At where t is the wall thickness and A is the area contained within the tube.
Thin walled sections of arbitrary and open cross sections have less torsional stiffness than
solid sections or tubular thin walled sections which allow shear to flow around the
section. In thin walled sections the shear flow is only able to develop within the thickness
of the walls and so the torsional stiffness comes from the sum of the stiffness of its parts:
J =
1
3
_
section
t
3
ds. This can be simplified to J ÷

bt
3
,3
_ _
, where t = Tt,J.
J for thick open sections are beyond the scope of this book, and must be calculated
empirically for the particular dimensions of a section. For non-square and circular shapes,
the effect of the warping of cross sections must be considered in addition to the elastic
effects set out above.
88 Structural Engineer's Pocket Book
Taut wires, cables and chains
The cables are assumed to have significant self-weight. Without any externally applied
loads, the horizontal component of the tension in the cable is constant and the maximum
tension will occur where the vertical component of the tension reaches a maximum. The
following equations are relevant where there are small deflections relative to the cable
length.
L Span length
h Cable sag
A Area of cable
ÁLs Cable elongation due to axial stress
C Length of cable curve
E Modulus of elasticity of the cable
s = h,L Sag ratio
w Applied load per unit length
V Vertical reaction
y Equation for the deflected shape
D Height of elevation
H Horizontal reaction
T
max
Maximum tension in cable
x Distance along cable
Uniformly loaded cables with horizontal chords
y =
4h(Lx ÷x
3
)
L
2
H =
wL
2
8h
V =
wL
2
T
max
= H

1 ÷
D
L
÷4s
_ _
2
_
C ÷ L sec y 1 ÷
8s
2
3sec
4
y
_ _
ÁL
s
÷
HL 1 ÷
16s
3 sec
4
y
_ _
AE
90 Structural Engineer's Pocket Book
Vibration
When using long spans and lightweight construction, vibration can become an important
issue. Human sensitivity to vibration has been shown to depend on frequency, amplitude
and damping. Vibrations can detract from the use of the structure or can compromise the
structural strength and stability.
Vibrations can be caused by wind, plant, people, adjacent building works, traffic, earth-
quakes or wave action. Structures will respond differently depending on their mass and
stiffness. Damping is the name given to the ability of the structure to dissipate the energy
of the vibrations ± usually by friction in structural and non-structural components. While
there are many sources of advice on vibrations in structures, assessment is not straight-
forward. In simple cases, structures should be designed so that their natural frequency is
greater than 4.5 Hz to help prevent the structure from being dynamically excitable.
Special cases may require tighter limits.
A simplified method of calculating the natural frequency of a structure (f in Hz) is related
to the static dead load deflection of the structure, where g is the acceleration due to
gravity, d is the static dead load deflection estimated by normal elastic theory, k is the
stiffness (k = L,EI), m is the total mass of the system, E is the modulus of elasticity, I is the
moment of inertia and L is the length of the member. This method can be used to check
the results of more complex analysis.
Member Estimate of natural
frequency, a
f
General rule for structures with
concentrated mass
f =
1
2p

EI
mL
4
_
For normal floors with span/depth ratios of 25 or less, there are unlikely to be any
vibration problems. Typically problems are encountered with steel and lightweight floors
with spans over about 8 m.
Source: Bolton, A (1978).
Basic and Shortcut Tools for Structural Analysis 91
5
Geotechnics
Geotechnics is the engineering theory of soils, foundations and retaining walls. This
chapter is intended as a guide which can be used alongside information obtained from
local building control officers, for feasibility purposes and for the assessment of site
investigation results. Scheme design should be carried out on the basis of a full site
investigation designed specifically for the site and structure under consideration.
The relevant codes of practice are:
.
BS 5930 for Site Investigation.
.
BS 8004 for Foundation Design and BS 8002 for Retaining Wall Design.
.
Eurocode 7 for Geotechnical Design.
The following issues should be considered for all geotechnical problems:
.
UK (and most international codes) use unfactored loads, while Eurocodes use factored
loads.
.
All values in this chapter are based on unfactored loads.
.
Engineers not familiar with site investigation tests and their implications, soil theory and
bearing capacity equations should not use the information in this chapter without using
the sources listed in `Further Reading' for information on theory and definitions.
.
The foundation information included in this chapter allows for simplified or idealized soil
conditions. In practice, soil layers and variability should be allowed for in the foundation
design.
.
All foundations must have an adequate factor of safety (normally g
f
=2 to 3) applied to
the ultimate bearing capacity to provide the allowable bearing pressure for design
purposes.
.
Settlement normally controls the design and allowable bearing pressures typically limit
settlement to 25 mm. Differential settlements should be considered. Cyclic or dynamic
loading can cause higher settlements to occur and therefore require higher factors of
safety.
.
Foundations in fine grained soils (such as clay, silt and chalk) need to be taken down to
a depth below which they will not be affected by seasonal changes in the moisture
content of the soil, frost action and the action of tree roots. Frost action is normally
assumed to be negligible from 450mm below ground level. Guidelines on trees and
shallow foundations in fine grained soils are covered later in the chapter.
.
Ground water control is key to the success of ground and foundation works and its
effects must be considered, both during and after construction. Dealing with water
within a site may reduce the water table of surrounding areas and affect adjoining
structures.
.
It is nearly always cheaper to design wide shallow foundations to a uniform and
predetermined depth, than to excavate narrow foundations to a depth which might
be variable on site.
Selection of foundations and retaining walls
The likely foundation arrangement for a structure needs to be considered so that an
appropriate site investigation can be specified, but the final foundation arrangement will
normally only be decided after the site investigation results have been returned.
Foundations for idealized structure and soil conditions
Foundations must always follow the building type ± i.e. a large-scale building needs large-
scale/deep foundations. Pad and strip foundations cannot practically be taken beyond
3m depth and these are grouped with rafts in the classification `shallow foundations',
while piles are called deep foundations. They can have diameters from 75 mm to
2000mm and be 5m to 100m in length. The smaller diameters and lengths tend to be
bored cast in-situ piles, while larger diameters and lengths are driven steel piles.
Idealized
extremes of
structure type
Idealized soil conditions
Firm,
uniform
soil in an
infinitely
thick
stratum
Firm
stratum
of soil
overlying
an infinitely
thick
stratum of
soft soil
Soft,
uniform
soil in an
infinitely
thick
stratum
High
water
table
and/or
made
ground
Soft
stratum
of soil
overlying
an infinitely
thick stratum
of firm soil
or rock
Light,
flexible
structure
Pad or
strip
footings
Pad or strip
footings
Friction
piles or
surface raft
Piles or
surface
raft
Bearing piles
or piers
Heavy rigid
structure
Pad or
strip
footings
Buoyant raft
or friction
piles
Buoyant raft
or friction
piles
Buoyant
raft
or piles
Bearing piles
or piers
Retaining walls for idealized site and soil conditions
Idealized site
conditions
Idealized soil types
Dry sand and
gravel
Saturated sand
and gravel
Clay and silt
Working space*
available
.
Gravity or
cantilever
retaining wall
.
Dewatering
during
construction of
gravity or
cantilever
retaining wall
.
Gravity or
cantilever
retaining wall
.
Reinforced
soil, gabion
or crib wall
Limited
working
space
.
King post or
sheet pile as
temporary
support
.
Sheet pile
and dewatering
.
King post or
sheet pile as
temporary
support
.
Contiguous
piled
wall
.
Secant bored
piled wall
.
Contiguous
piled wall
.
Diaphragm
wall
.
Diaphragm
wall
.
Soil nailing
.
Soil nailing
.
Diaphragm wall
Limited working
space and special
controls on ground
movements
.
Contiguous
piled wall
.
Secant bored
piled wall
.
Contiguous
piled wall
.
Diaphragm
wall
.
Diaphragm
wall
.
Diaphragm
wall
* Working space available to allow the ground to be battered back during wall construction.
Geotechnics 93
Site investigation
In order to decide on the appropriate form of site investigation, the engineer must have
established the position of the structure on the site, the size and form of the structure,
and the likely foundation loads.
BS 5390: Part 2 suggests that the investigation is taken to a depth of 1.5 times the width
of the loaded area for shallow foundations. A loaded area can be defined as the width of
an individual footing area, the width of a raft foundation, or the width of the building (if
the foundation spacing is less than three times the foundation breadth). An investigation
must be conducted to prove bedrock must be taken down 3 m beyond the top of the
bedrock to ensure that rock layer is sufficiently thick.
Summary of typical site investigation requirements for
idealized soil types
Soil type Type of geotechnical work
Excavations Shallow footings
and rafts
Deep foundations
and piles
Sand
.
Permeability for
dewatering and stability
of excavation bottom
.
Shear strength
for bearing capacity
calculations
.
Test pile for assessment
of allowable bearing
capacity and settlements
.
Shear strength
for loads on
retaining structures
and stability of
excavation bottom
.
Site loading
tests for
assessment of
settlements
.
Deep
boreholes
to probe
zone of
influence
of piles
Clay
.
Shear strength
for loads
on retaining
structure and
stability of
excavation
bottom
.
Shear
strength
for bearing
capacity
calculations
.
Long-term
test pile for
assessment
of allowable
bearing capacity
and settlements
.
Sensitivity
testing to assess
strength and
stability and
the possibility
of reusing material
as backfill
.
Consolidation
tests for
assessment of
settlements
.
Shear
strength
and sensitivity
testing to
assess bearing
capacity and
settlements
.
Moisture
content and
plasticity tests
to predict
heave potential
and effects of
trees
.
Deep boreholes
to probe zone
of influence
of piles
94 Structural Engineer's Pocket Book
Soil classification
Soil classification is based on the sizes of particles in the soil as divided by the British
Standard sieves.
Sieve size
Description Silt Sand Gravel Cobbles Boulders
0.002 0.006 0.020 0.06 0.2 0.6 2.0 6.0 20 60 200
mm
Soil description by particle size
As soils are not normally uniform, standard descriptions for mixed soils have been defined
by BS 5930. The basic components are boulders, cobbles, gravel, sand, silt and clay and
these are written in capital letters where they are the main component of the soil.
Typically soil descriptions are as follows:
Slightly sandy
GRAVEL
up to 5% sand Sandy GRAVEL 5%±20% sand
Very sandy
GRAVEL
20%±50% sand GRAVEL /SAND equal proportions
Very gravelly
SAND
20%±50% gravel Slightly gravelly
SAND
up to 5% gravel
Slightly silty
SAND (or
GRAVEL)
up to 5% silt Silty SAND
(or GRAVEL)
5%±15% silt
Very silty SAND
(or GRAVEL)
15%±35% silt Slightly clayey
SAND (or GRAVEL)
up to 5% clay
Clayey SAND
(or GRAVEL)
5%±15% clay Very clayey SAND
(or GRAVEL)
15%±35% clay
Sandy SILT
(or CLAY)
35%±65% sand Gravelly SILT
(or CLAY)
35%±65% gravel
Very coarse over 50% cobbles
and boulders
Soil description by consistency
Homogeneous A deposit consisting of one soil type.
Heterogeneous A deposit containing a mixture of soil types.
Interstratified A deposit containing alternating layers, bands or
lenses of different soil types.
Weathered Coarse soils may contain weakened particles and/or
particles sorted according to their size. Fine soils
may crumble or crack into a `column' type structure.
Fissured clay Breaks into multifaceted fragments along fissures.
Intact clay Uniform texture with no fissures.
Fibrous peat Recognizable plant remains present, which retains
some strength.
Amorphous peat Uniformtexture, with no recognizable plant remains.
Geotechnics 95
Typical soil properties
The presence of water is critical to the behaviour of soil and the choice of shear strength
parameters (internal angle of shearing resistance, f and cohesion, c) are required for
geotechnical design.
If water is present in soil, applied loads are carried in the short term by pore water
pressures. For granular soils above the water table, pore water pressures dissipate almost
immediately as the water drains away and the loads are effectively carried by the soil
structure. However, for fine grained soils, which are not as free draining, pore water
pressures take much longer to dissipate. Water and pore water pressures affect the
strength and settlement characteristics of soil.
The engineer must distinguish between undrained conditions (short-term loading, where
pore water pressures are present and design is carried out for total stresses on the basis of
f
u
and C
u
) and drained conditions (long-term loading, where pore water pressures have
dissipated and design is carried out for effective stresses on the basis of f' and c').
Drained conditions, f' >0
Failure
envelope
Effective
stress
circle
Over
consolidated
clays
c
′
=0
generally
c
′
>0
Direct stress
σ
φ
′
Shear
stress
τ
96 Structural Engineer's Pocket Book
Approximate correlation of properties for drained granular soils
Description SPT*
N
blows
Effective internal
angle of shearing
resistance
f'
Bulk unit
weight
g
bulk
kN/m
3
Dry unit
weight
g
dry
kN/m
3
Very loose 0±4 26±28 <16 <14
Loose 4±10 28±30 16±18 14±16
Medium dense 10±30 30±36 18±19 16±17
Dense 30±50 36±42 19±21 17±19
Very dense 50 42±46 21 19
* An approximate conversion from the standard penetration test to the Dutch cone penetration test:
C
r
~ 400NkN/m
2
.
For saturated, dense, fine or silty sands, measured N values should be reduced by:
N=15÷0.5(N÷15).
Approximate correlation of properties for drained cohesive soils
The cohesive strength of fine grained soils normally increases with depth. Drained shear
strength parameters are generally obtained from very slow triaxial tests in the laboratory.
The effective internal angle of shearing resistance, f', is influenced by the range and
distribution of fine particles, with lower values being associated with higher plasticity. For
a normally consolidated clay the effective (or apparent) cohesion, c', is zero but for an
overconsolidated clay it can be up to 30 kN/m
2
.
Soil description Typical
shrink-
ability
Plasticity
index
PI
%
Bulk unit
weight
g
bulk
kN/m
3
Effective
internal angle
of shearing
resistance
f
/
Clay High 35 16±22 18±24
Silty clay Medium 25±35 16±20 22±26
Sandy clay Low 10±25 16±20 26±34
Geotechnics 97
Undrained conditions, f
u
=0
Approximate correlation of properties for undrained cohesive soils
Description Undrained shear strength
C
u
kN/m
2
Bulk unit weight
g
bulk
kN/m
3
Very stiff and hard clays 150 19±22
Stiff clays 100±150
Firm to stiff clays 75±100 17±20
Firm clays 50±75
Soft to firm clays 40±50
Soft clays and silts 20±40 16±19
Very soft clays and silts 20
It can be assumed that C
u
~ 4.5N if the clay plasticity index is greater than 30, where N is
the number of Standard Penetration Test (SPT) blows.
Typical values of Californian Bearing Ratio (CBR)
Description CBR
Soft clay 0.5±1
Firm clay 1.5±2
Stiff clay/loose sand 3±5
Compact sand 10±15
Loose gravel 20±25
Compact gravel 40±50
Shear
stress
τ Effective
stress
circle Failure
envelope
Direct stress σ
C
u
C
u
>0
φ
u
=0
98 Structural Engineer's Pocket Book
Typical angle of repose for selected soils
The angle of repose is very similar to, and often confused with, the internal angle of
shearing resistance. The internal angle of shearing resistance is calculated from laboratory
tests and indicates the theoretical internal shear strength of the soil for use in calculations
while the angle of repose relates to the expected field behaviour of the soil. The angle of
repose indicates the slope which the sides of an excavation in the soil might be expected
to stand at. The values given below are for short-term, unweathered conditions.
Soil type Description Typical
angle of
repose
Description Typical
angle of
repose
Top soil Loose and dry 35±40 Loose and
saturated
45
Loam Loose and dry 40±45 Loose and
saturated
20±25
Peat Loose and dry 15 Loose and
saturated
45
Clay/Silt Firm to
moderately firm
17±19 Puddle clay 15±19
Sandy clay 15 Silt 19
Loose and wet 20±25 Solid naturally moist 40±50
Sand Compact 35±40 Loose and dry 30±35
Sandy gravel 35±45 Saturated 25
Gravel Uniform 35±45 Loose shingle 40
Sandy compact 40±45 Stiff boulder/
hard shale
19±22
Med coarse
and dry
30±45 Med coarse
and wet
25±30
Broken
rock
Dry 35 Wet 45
Geotechnics 99
Preliminary sizing
Typical allowable bearing pressures under static loads
Description Safe bearing
capacity
1
kN/m
2
Field description/notes
Igneous rocks
and gneisses
5000 Footings on unweathered rock
Limestones and
hard sandstones
up to 3000
Schists and slates up to 2000
Shales and mudstones up to 1000
Hard block chalk up to 600 Beware of sink holes and hollowing
as a result of water flow
Very stiff and hard clays 300±600 Requires pneumatic spade
for excavation but can be
indented by the thumbnail
Stiff clays 150±300 Hand pick ± cannot be
moulded in hand but can be
indented by the thumb
Firm clays 75±150 Can be moulded with firm
finger pressure
Soft clays and silts <75 Easily moulded with firm
finger pressure
Very soft clays and silts nil Extrudes between fingers
when squeezed
Compact gravel and
sandy gravel
2
600 Requires pneumatic tools
for excavation
Medium dense gravel
and sandy gravel
2
200±600 Hand pick ± resistance to
shovelling
Loose gravel and sandy gravel
2
<200 Small resistance to shovelling
Compact sand
2
300÷ Hand pick ± resistance
to shovelling
Medium dense sand
2
100±300 Hand pick ± resistance to shovelling
Loose sand
2
<100 Small resistance to shovelling
Firm organic
material/medieval fill
20±40 Can be indented by thumbnail.
Only suitable for small
scale-buildings where settlements
may not be critical
Unidentifiable
made ground
25±50 Bearing values depend on the
likelihood of voids and the
compressibility of the made ground
Springy organic material/peats nil Very compressible and open
structure
Plastic organic material/peats nil Can be moulded in the hand
and smears the fingers
NOTES:
1. This table should be read in accordance with the limitations of BS 8004.
2. Values for granular soil assume that the footing width, B, is not less than 1m and that the water table
is more than B below the base of the foundation.
Source: BS 8004: 1986.
100 Structural Engineer's Pocket Book
Quick estimate design methods for shallow foundations
General equation for allowable bearing capacity
after Brinch Hansen
Factor of safety against bearing capacity failure,
g
f
=2.0 to 3.0, q
/
o
is the effective over-
burden pressure, g is the unit weight of the soil, B is the width of the foundation, c is the
cohesion (for the drained or undrained case under consideration) and N
c
, N
q
and N
g
are
shallow bearing capacity factors.
Strip footings: q
allowable
=
cN
c
÷ q
/
o
N
q
÷ 0.5gBN
g
g
f
Pad footings: q
allowable
=
1.3cN
c
÷ q
/
o
N
q
÷ 0.4gBN
g
g
f
Approximate values for the bearing capacity factors N
c
, N
q
and N
g
are set out below in
relation to f.
Internal angle of shear Bearing capacity factors*
f
N
c
N
q
N
g
0 5.0 1.0 0.0
5 6.5 1.5 0.0
10 8.5 2.5 0.0
15 11.0 4.0 1.4
20 15.5 6.5 3.5
25 21.0 10.5 8.0
30 30.0 18.5 17.0
35 45.0 34.0 40.0
40 75.0 65.0 98.0
* Values from charts by Brinch Hansen (1961).
Simplified equations for allowable bearing capacity after Brinch
Hansen
For very preliminary design, Terzaghi's equation can be simplified for uniform soil in thick
layers.
Spread footing on clay
q
allowable
=2C
u
Spread footing on undrained cohesive soil (g
f
=2.5)
Spread footing on gravel
q
allowable
=10N Pad footing on dry soil (g
f
=3)
q
allowable
=7N Strip footing on dry soil (g
f
=3)
q
wet allowable
=q
allowable
/2 Spread foundation at or below the water table
Geotechnics 101
Quick estimate design methods for deep foundations
Concrete and steel pile capacities
Concrete piles can be cast in situ or precast, prestressed or reinforced. Steel piles are used
where long or lightweight piles are required. Sections can be butt welded together and
excess can be cut away. Steel piles have good resistance to lateral forces, bending and
impact, but they can be expensive and need corrosion protection.
Typical maximumallowable pile capacities can be 300 to 1800kNfor bored piles (diameter 300
to600mm), 500to2000kNfor drivenpiles (275to400mmsquare precast or 275to2000mm
diameter steel), 300 to 1500kN for continuous flight auger (CFA) piles (diameter 300 to
600mm) and 50 to 500kN for mini piles (diameter 75 to 280mm and length up to 20m).
The minimumpile spacing achievable is normally about three diameters between the pile faces.
2000
1800
1600
1400
1200
1000
800
600
400
200
0
5 10 15 20 25 30
P
i
l
e

c
a
p
a
c
i
t
y
900 φ
750 φ
600 φ
450 φ
300 φ
150 φ
Pile length (m)
Working pile loads for CFA piles in cohesive soil (C
u
=50)
Working pile loads for CFA piles in cohesive soil (C
u
=100)
Geotechnics 103
Single bored piles in clay
Q
allow
=
N
c
A
b
c
base
g
f base
÷
a"cA
s
g
f shaft
Where A
b
is the area of the pile base, A
s
is the surface area of the pile shaft in the clay, "c is
the average value of shear strength over the pile length and is derived from undrained
triaxial tests, where a =0.3 to 0.6 depending on the time that the pile boring is left open.
Typically a=0.3 for heavily fissured clay and a=0.45±0.5 for firm to stiff clays (e.g.
London clay). N
c
=9 where the embedment of the tip of the pile into the clay is more than
five diameters. The factors of safety are generally taken as 2.5 for the base and 3.0 for the
shaft.
Group action of bored piles in clay
The capacity of groups of piles can be as little as 25 per cent of the collective capacity of
the individual piles.
A quick estimate of group efficiency:
E =1 ÷ tan
÷1
D
S
_ _
[m(n ÷1) ÷n(m÷1)[
90mn
Where D is the pile diameter, S is the pile spacing and m and n represent the number of
rows in two directions of the pile group.
Negative skin friction
Negative skin friction occurs when piles have been installed through a compressible
material to reach firm strata. Cohesion in the soft soil will tend to drag down on the
piles as the soft layer consolidates and compresses causing an additional load on the pile.
This additional load is due to the weight of the soil surrounding the pile. For a group of
piles a simplified method of assessing the additional load per pile can be based on the
volume of soil which would need to be supported on the pile group.
Q
skin friction
= AHg,N
p
where A is the area of the pile group, H is the thickness of the
layer of consolidating soil or fill which has a bulk density of g, and N
p
is the number of
piles in the group. The chosen area of the pile group will depend on the arrangement of
the piles and could be the area of the building or part of the building. This calculation can
be applied to individual piles, although it can be difficult to assess how much soil could be
considered to contribute to the negative skin friction forces.
104 Structural Engineer's Pocket Book
Piles in granular soil
Although most methods of determining driven pile capacities require information on the
resistance of the pile during driving, capacities for both driven and bored piles can be
estimated by the same equation. The skin friction and end bearing capacity of bored piles
will be considerably less than driven piles in the same soil as a result of loosening caused by
the boring and design values of g, N and k
s
tanc should be selected for loose conditions.
Q
allow
=
N
+
q
A
b
q
/
o
÷A
s
q
/
o mean
k
s
tand
g
f
Where N
+
q
is the pile bearing capacity factor based on the work of Berezantsev, A
b
is the
area of the pile base, A
s
is the surface area of the pile shaft in the soil, q
o
/
is the effective
overburden pressure, k
s
is the horizontal coefficient of earth pressure, k
o
is the coefficient
of earth pressure at rest, d is the angle of friction between the soil and the pile face, f
/
is
the effective internal angle of shearing resistance and the factor of safety, g
f
=2.5 to 3.
Typical values of N
q
*
Pile length
Pile diameter
f 5 20
25 16 11 7
30 29 24 20
35 69 53 45
40 175 148 130
* Berezantsev (1961) values from charts for N
q
based on f calculated from uncorrected N values.
Typical values of d and k
s
for sandy soils can therefore be determined based on work by
Kulhawy (1984) as follows:
Pile face/soil type Angle of pile/soil friction
d/f
/
Smooth (coated) steel/sand 0.5±0.9
Rough (corrugated) steel/sand 0.7±0.9
Cast in place concrete/sand 1.0
Precast concrete/sand 0.8±1.0
Timber/sand 0.8±0.9
Installation and pile type Coefficients of horizontal soil
stress/earth pressure at rest k
s
/k
o
Driven piles large displacement 1.00±2.00
Driven piles small displacement 0.75±1.25
Bored cast in place piles 0.70±1.00
Jetted piles 0.50±0.70
Although pile capacities improve with depth, it has been found that at about 20 pile
diameters, the skin friction and base resistances stop increasing and `peak' for granular
soils. Generally the peak value for base bearing capacity is 110 000kN/m
2
for a pile length
of 10 to 20 pile diameters and the peak values for skin friction are 10 kN/m
2
for loose
granular soil, 10 to 25 kN/m
2
for medium dense granular soil, 25 to 70 kN/m
2
for dense
granular soil and 70 to 110kN/m
2
for very dense granular soil.
Source: Kulhawy, F.H (1984). Reproduced by permission of the ASCE.
Geotechnics 105
Pile caps
Pile caps transfer the load from the superstructure into the piles and take up tolerances on
the pile position (typically ±75 mm). The pile cap normally projects 150mm beyond the
pile face and if possible, only one depth of pile cap should be used on a project to
minimize cost and labour. The Federation of Piling Specialists suggest the following pile
cap thicknesses which generally will mean that the critical design case will be for the sum
of all the pile forces to one side of the cap centre line, rather than punching shear:
Pile
diameter (mm)
300 350 400 450 500 550 600 750
Pile cap
depth (mm)
700 800 900 1000 1100 1200 1400 1600
106 Structural Engineer's Pocket Book
Retaining walls
Rankine's theory on lateral earth pressure is most commonly used for retaining wall design,
but Coulomb's theory is easier to apply for complex loading conditions. The most difficult
part of Rankine's theory is the appropriate selection of the coefficient of lateral earth
pressure, which depends on whether the wall is able to move. Typically where sufficient
movement of a retaining wall is likely and acceptable, `active' and `passive' pressures can be
assumed, but where movement is unlikely or unacceptable, the earth pressures should be
considered `at rest'. Active pressure will be mobilized if the wall moves 0.25±1 per cent of
the wall height, while passive pressure will require movements of 2±4 per cent in dense
sand or 10±15 per cent in loose sand. As it is normally difficult to assume that passive
pressure will be mobilized, unless it is absolutely necessary for stability (e.g. embedded
walls), the restraining effects of passive pressures are often ignored in analysis. The main
implications of Rankine's theory are that the engineer must predict the deflected shape, to
be able to predict the forces which will be applied to the wall.
Rankine's theory assumes that movement occurs, that the wall has a smooth back, that the
retained ground surface is horizontal and that the soil is conhesionless, so that: s
h
=ks
v
For soil at rest, k =k
o
, for active pressure, k =k
a
and for passive pressure, k =k
p
.
k
o
~ 1 ÷sin f k
a
=
(1 ÷sin f)
(1 ÷sin f)
k
p
=
1
k
a
=
(1 ÷sin f)
(1 ÷sin f)
For cohesive soil, k
o
should be factored by the overconsolidation ratio,
OCR =

pre-consolidation pressure
effective overburden pressure
_
.
Typical k
o
values are 0.35 for dense sand, 0.6 for loose sand, 0.5 to 0.6 for normally
consolidated clay and 1.0 to 2.8 for overconsolidated clays such as London clay. The value
of k
o
depends on the geological history of the soil and should be obtained from a
geotechnical engineer.
Rankine's theory can be adapted for cohesive soils, which can shrink away from the wall
and reduce active pressures at the top of the wall as a tension `crack' forms. Theoretically
the soil pressures over the height of the tension crack can be omitted from the design, but
in practice the crack is likely to fill with water, rehydrate the clay and remobilize the lateral
pressure of the soil. The height of crack is h
c
=2c
/
,(g

k
a
_
) for drained conditions and
h
c
=2C
u
,g for undrained conditions.
Geotechnics 107
Preliminary sizing of retaining walls
Gravity retaining walls ± Typically have a base width of about 60±80 per cent of the
retained height.
Propped embedded retaining walls ± There are 16 methods for the design of these
walls depending on whether they are considered flexible (sheet piling) or rigid (concrete
diaphragm). A reasonable approach is to use BS 8002 Free Earth Support Method which
takes moments about the prop position, followed by the Burland & Potts Method as a
check. Any tension crack height is limited to the position of the prop.
Embedded retaining walls ± Must be designed for fixed earth support where passive
pressures are generated on the rear of the wall, at the toe. An approximate design
method is to design the wall with free earth support by the same method as the propped
wall but with moments taken at the foot of the embedded wall, before adding 20 per
cent extra depth as an estimate of the extra depth required for the fixed earth condition.
108 Structural Engineer's Pocket Book
Trees and shallow foundations
Trees absorb water from the soil which can cause consolidation and settlements in fine
grained soils. Shallow foundations in these conditions may be affected by these settle-
ments and the National House Building Council (NHBC) publish guidelines on the depth
of shallow foundations on silt and clay soils to take the foundation to a depth beyond the
zone of influence of tree roots. The information reproduced here is current in 2002, but
the information may change over time and amendments should be checked with NHBC.
The effect depends on the plasticity index of the soil, the proximity of the tree to the
foundation, the mature height of the tree and its water demand. The following suggested
minimum foundation depths are based on the assumption that low water demand trees
are located 0.2 times the mature height from the building, moderate water demand trees
at 0.5 times the mature height and high water demand trees at 1.25 times the mature
height of the tree. Where the plasticity index of the soil is not known, assume high
plasticity.
Plasticity index,
PI =Liquid limit ± Plastic limit
Minimum foundation
depth with no trees
m
Low 10±20% 0.75
Medium 10±40% 0.9
High 40% 1.0
Source: NHBC (2002). The information may change at any time and revisions should be
checked with NHBC.
Geotechnics 109
Water demand and mature height of selected UK trees
The following common British trees are classified as having high, moderate or low water
demand. Where the tree cannot be identified, assume high water demand.
Water
demand
Broad leaved
trees
Conifers Broad leaf
orchard trees
Species Mature
height*
Species Mature
height*
Species Mature
Height*
Species Mature
height*
m m m m
High Elm 18±24 Poplar 25±28 Cypress 18±20
Eucalyptus 18 Willow 16±24
Oak 16±24
Moderate Acacia
false
18 Laburnum 12 Cedar 20 Apple 9
Alder 18 Lime 22 Douglas
fir
20 Cherry 15
Ash 23 Maple 8±18 Pine 20 Pear 12
Bay laurel 10 Mountain
ash
11 Spruce 18 Plum 10
Blackthorn 8 Plane 26 Wellingtonia 30
Cherry 9±17 Sycamore 22 Yew 12
Hawthorn 10 Tree of
heaven
20
Honey
locust
14 Walnut 18
Hornbeam 17 Whitebeam 12
Horse
chestnut
20
Low Beech 20 Magnolia 9
Birch 14 Mulberry 9
Holly 12
* For range of heights within species, see the full NHBC source table for full details.
Source: NHBC (2002). The information may change at any time and revisions should be
checked with NHBC.
110 Structural Engineer's Pocket Book
Suggested depths for foundations on cohesive soil
If D is the distance between the tree and the foundation, and H is the mature height of
the tree, the following three charts (based on soil shrinkability) will estimate the required
foundation depth for different water demand classifications. The full NHBC document
allows for a reduction in the foundation depth for climatic reasons, for every 50 miles
from the South-East of England.
Suggested depths for foundations on highly shrinkable soil
Source: NHBC (2002). The information may change at any time and revisions should be
checked with NHBC.
0
0.2 0.4 0.6
D/H
0.8 1.0 1.2
Minimum depth 1.0 m
0.5
1.0
1.5
L
o
w
M
o
d
e
r
a
t
e
M
o
d
e
r
a
t
e
H
i
g
h
H
i
g
h
D
e
p
t
h

(
m
)
)
2.0
2.5
3.0
3.5
Broad leaf water demand
Coniferous water demand
Suggested depths for foundations on medium shrinkable soil
Suggested depths for foundations on low shrinkability soil
Source: NHBC (2002). The information may change at any time and revisions should be
checked with NHBC.
112 Structural Engineer's Pocket Book
Contaminated land
Contamination can be present as a result of pollution from previous land usage or
movement of pollutants from neighbouring sites by air or ground water. The main
categories of contamination are chemical, biological (pathological bacteria) and physical
(radioactive, flammable materials, etc.).
The Environmental Protection Act 1990 (in particular Part IIA) is the primary legislation
covering the identification and remediation of contaminated land. The Act defines con-
tamination as solid, liquid, gas or vapour which might cause harm to `targets'. This can
mean harm to the health of living organisms or property, or other interference with
ecological systems. The contamination can be on, in or under the land. The Act applies if
the contamination is causing, or will cause, significant harm or results in the pollution of
controlled waters including coastal, river and ground water. In order to cause harm the
pollution must have some way (called a `pathway') of reaching the `target'. The amount of
harmwhich can be caused by contamination will depend on the proposed use for the land.
Remediation of contaminated land can remove the contamination, reduce its concentra-
tions below acceptable levels, or remove the `pathway'.
The 1990 Act set up a scientific framework for assessing the risks to human health from
land contamination. This has resulted in Contaminated Land Exposure Assessment (CLEA)
and development of Soil Guideline Values for residential, allotment or industrial/commer-
cial land use. Where contaminant concentration levels exceed the Soil Guideline Values,
further investigation and/or remediation is required. Reports are planned for a total of
55 contaminants and some are available on the Environment Agency website. Without
the full set, assessment is frequently made using Guideline Values from the Netherlands.
Other frequently mentioned publications are Kelly and the now superseded ICRCL list.
Zero Environment has details of the ICRCL, Kelly and Dutch lists on its website.
Before developing a `brownfield site' (i.e. a site which has previously been used) a desk
study on the history of the site should be carried out to establish its previous uses and
therefore likely contaminants. Sampling should then be used to establish the nature and
concentration of any contaminants. Remedial action may be dictated by law, but should
be feasible and economical on the basis of the end use of the land.
Geotechnics 113
Common sources of contamination
Specific industries can be associated with particular contaminants and the site history is
invaluable in considering which soil tests to specify. The following list is a summary of
some of the most common sources of contamination.
Common contaminants Possible sources of contaminants
`Toxic or heavy metals' (cadmium, lead, arsenic,
mercury, etc.)
Metal mines; iron and steelworks; foundries and
electroplating
`Safe' metals (copper, nickel, zinc, etc.) Anodizing and galvanizing; engineering/ship/scrap
yards
Combustible materials such as coal and coke
dust
Gas works; railways; power stations; landfill sites
Sulphides, chlorides, acids and alkalis Made-up ground
Oily or tarry deposits and phenols Chemical refineries; chemical plants; tar works
Asbestos Twentieth century buildings
Effects of contaminants
Effect of contaminant Typical contaminants
Toxic /narcotic gases and vapours Carbon monoxide or dioxide, hydrogen sulphide,
hydrogen cyanide, toluene, benzene
Flammable and explosive gases Acetylene, butane, hydrogen sulphide, hydrogen,
methane, petroleum hydrocarbons
Flammable liquids and solids Fuel oils, solvents; process feedstocks, intermediates
and products
Combustible materials Coal residues, ash timber, variety of domestic
commercial and industrial wastes
Possible self-igniting materials Paper, grain, sawdust ± microbial degradation of large
volumes if sufficiently damp
Corrosive substances Acids and alkalis; reactive feedstocks, intermediates and
products
Zootoxic metals and their salts Cadmium, lead, mercury, arsenic, beryllium and copper
Other zootoxic metals Pesticides, herbicides
Carcinogenic substances Asbestos, arsenic, benzene, benzo(a)pyrene
Substances resulting in skin damage Acids, alkalis, phenols, solvents
Phytotoxic metals Copper, zinc, nickel, boron
Reactive inorganic salts Sulphate, cyanide, ammonium, sulphide
Pathogenic agents Anthrax, polio, tetanus, Weils
Radioactive substances Waste materials from hospitals, mine workings, power
stations, etc.
Physically hazardous materials Glass, blades, hospital wastes ± needles etc.
Vermin and associated pests Rats, mice and cockroaches (contribute to pathogenic
agents)
(Where zootoxic means toxic to animals and phytotoxic means toxic to plants.)
114 Structural Engineer's Pocket Book
Site investigation and sampling
Once a desk study has been carried out and the most likely contaminants are known, an
assessment must be carried out to establish the risks associated with the contaminants
and the proposed land use. These two factors will determine the maximum concentra-
tions of contaminants which will be acceptable. These maximum concentrations are the
Soil Guideline Values published by the Department for the Environment, Food and Rural
Affairs (DEFRA) as part of the CLEA range of documents.
Once the soil guideline or trigger values have been selected, laboratory tests can be
commissioned to discover if the selected soil contaminants exist, as well as their concen-
tration and their distribution over the site. Reasonably accurate information can be
gathered about the site using a first stage of sampling and testing to get a broad picture
and a second stage to define the extents of localized areas of contamination.
Sampling on a rectangular grid with cores of 100mm diameter, it is difficult to assess
how many samples might be required to get a representative picture of the site. British
Standards propose 25 samples per 10 000m
2
which is only 0.002 per cent of the site
area. This would only give a 30 per cent confidence of finding a 100m
2
area of
contamination on the site, while 110 samples would give 99 per cent confidence. It is
not easy to balance the cost and complexity of the site investigations and the cost of any
potential remedial work, without an appreciation of the extent of the contamination on
the site!
Geotechnics 115
Remediation techniques
There are a variety of techniques available depending on the contaminant and target user.
The chosen method of treatment will not necessarily remove all of a particular contami-
nant from a site as in most circumstances it may be sufficient to reduce the risk to below
the predetermined trigger level. In some instances it may be possible to change the
proposed layout of a building to reduce the risk involved. However, if the site report
indicates that the levels of contaminant present in the soil are too high, four main
remediation methods are available:
Excavation
Excavation of contaminated soil for specialist disposal or treatment (possibly in a specialist
landfill site) and reconstruction of the site with clean fill material. This is expensive and the
amount of excavated material can sometimes be reduced, by excavating down to a
limited `cut-off' level, before covering the remaining soil with a barrier and thick granular
layer to avoid seepage/upward migration. Removal of soil on restricted sites might affect
existing, adjacent structures.
Blending
Clean material is mixed into the bulk of the contaminated land to reduce the overall
concentrations taking the test samples below trigger values. This method can be cost
effective if some contaminated soil is removed and replaced by clean imported fill, but it is
difficult to implement and the effects on adjacent surfaces and structures must be taken
into account.
Isolation
Isolation of the proposed development from the contaminants can be attempted by
displacement sheet piling, capping, horizontal/vertical barriers, clay barriers, slurry
trenches or jet grouting. Techniques should prevent contaminated soil from being
brought out of the ground to contaminate other areas.
Physical treatment
Chemical or biological treatment of the soil so that the additives bond with and reduce
the toxicity of, or consume, the contaminants.
116 Structural Engineer's Pocket Book
6
Timber and Plywood
Commercial timbers are defined as hardwoods and softwoods according to their botani-
cal classification rather than their physical strength. Hardwoods are from broad leaved
trees which are deciduous in temperate climates. Softwoods are from conifers, which are
typically evergreen with needle shaped leaves.
Structural timber is specified by a strength class which combines the timber species and
strength grade. Strength grading is the measurement or estimation of the strength
of individual timbers, to allow each piece to be used to its maximum efficiency. This
can be done visually or by machine. The strength classes referred to in Eurocode 5 and BS
5268 are C14 to C40 for softwoods (C is for coniferous) and D30 to D70 for hardwoods
(D is for deciduous). The number refers to the ultimate bending strength in N/mm
2
before
application of safety factors for use in design. The Eurocodes use Limit State Design with
factored design loads. The British Standards use permissible stresses and grade stresses
are modified by load factors according to the design conditions. C16 is the most
commonly available softwood, followed by the slightly stronger C24. Specification of
C24 should generally be accompanied by checks to confirm that it has actually been used
on site in preference to the more readily available C16.
Timber products
Wood-based sheet materials are the main structural timber products, containing sub-
stantial amounts of wood in the form of strips, veneers, chips, flakes or fibres. These
products are normally classified as:
Laminated panel products ± Plywood, laminated veneered lumber (LVL) and glue
laminated timber (glulam) for structural use. Made out of laminations 2 to 43 mm thick
depending on the product.
Particleboard ± Chipboard, orientated strandboard (OSB) and wood-wool. Developed
to use forest thinnings and sawmill waste to create cheap panelling for building applica-
tions. Limited structural uses.
Fibreboard ± Such as hardboard, medium density fibreboard (MDF). Fine particles
bonded together with adhesive to form general, non-structural, utility boards.
Summary of material properties
Density 1.2 to 10.7kN/m
3
. Softwood is normally assumed to be between 4 and 6 kN/m
3
.
Moisture content After felling, timber will lose moisture to align itself with atmo-
spheric conditions and becomes harder and stronger as it loses water. In the UK the
atmospheric humidity is normally about 14%. Seasoning is the name of the controlled
process where moisture content is reduced to a level appropriate for the timber's
proposed use. Air seasoning within the UK can achieve a moisture content of 17±23%
in several months for softwood, and over a period of years for hardwoods. Kiln drying can
be used to achieve the similar moisture contents over several days for softwoods or two to
three weeks for hardwoods.
Moisture content should be lower than 20% to stop fungal attack.
Shrinkage Shrinkage occurs as a result of moisture loss. Typical new structural soft-
wood will reduce in depth across the grain by as much as 3±4% once it is installed in a
heated environment. Shrinkage should be allowed for in structural details.
BS 5268: Part 2 sets out Service Classes 1, 2 and 3 which define timber as having moisture
contents of <12%, <20% and >20% respectively.
118 Structural Engineer's Pocket Book
Timber section sizes
Selected timber section sizes and section properties
Timber over the standard maximum length, of about 5.5m, is more expensive and must
be pre-ordered.
Basic size* Area Z
xx
Z
YY
I
XX
I
YY
r
xx
r
yy
D B 10
2
mm
2
10
3
mm
3
10
3
mm
3
10
6
mm
4
10
6
mm
4
mm mm
mm mm
100 38 38 63.3 24.1 3.17 0.46 28.9 11.0
100 50 50 83.3 41.7 4.17 1.04 28.9 14.4
100 63 63 105.0 66.2 5.25 2.08 28.9 18.2
100 75 75 125.0 93.8 6.25 3.52 28.9 21.7
100 100 100 166.7 166.7 8.33 8.33 28.9 28.9
150 38 57 142.5 36.1 10.69 0.69 43.3 11.0
150 50 75 187.5 62.5 14.06 1.56 43.3 14.4
150 63 94 236.3 99.2 17.72 3.13 43.3 18.2
150 75 112 281.3 140.6 21.09 5.27 43.3 21.7
150 100 150 375.0 250.0 28.13 12.50 43.3 28.9
150 150 225 562.5 562.5 42.19 42.19 43.3 43.3
175 38 66 194.0 42.1 16.97 0.80 50.5 11.0
175 50 87 255.2 72.9 22.33 1.82 50.5 14.4
175 63 110 321.6 115.8 28.14 3.65 50.5 18.2
175 75 131 382.8 164.1 33.50 6.15 50.5 21.7
200 38 76 253.3 48.1 25.33 0.91 57.7 11.0
200 50 100 333.3 83.3 33.33 2.08 57.7 14.4
200 63 126 420.0 132.3 42.00 4.17 57.7 18.2
200 75 150 500.0 187.5 50.00 7.03 57.7 21.7
200 100 200 666.7 333.3 66.67 16.67 57.7 28.9
200 150 300 1000.0 750.0 100.00 56.25 57.7 43.3
200 200 400 1333.3 1333.3 133.33 133.33 57.7 57.7
225 38 85 320.6 54.2 36.07 1.03 65.0 11.0
225 50 112 421.9 93.8 47.46 2.34 65.0 14.4
225 63 141 531.6 148.8 59.80 4.69 65.0 18.2
225 75 168 632.8 210.9 71.19 7.91 65.0 21.7
250 50 125 520.8 104.2 65.10 2.60 72.2 14.4
250 75 187 781.3 234.4 97.66 8.79 72.2 21.7
250 100 250 1041.7 416.7 130.21 20.83 72.2 28.9
250 250 625 2604.2 2604.2 325.52 325.52 72.2 72.2
300 50 150 750.0 125.0 112.50 3.13 86.6 14.4
300 75 225 1125.0 281.3 168.75 10.55 86.6 21.7
300 100 300 1500.0 500.0 225.00 25.00 86.6 28.9
300 150 450 2250.0 1125.0 337.50 84.38 86.6 43.3
300 300 900 4500.0 4500.0 675.00 675.00 86.6 86.6
* Under dry exposure conditions.
Source: BS 5268: Part 2: 1991.
Tolerances on timber cross sections
BS EN 336 sets out the customary sizes of structural timber. Class 1 timbers are `sawn'
and Class 2 timbers are `planed'. The permitted deviations for tolerance Class 1
are ÷1 mm to ÷3 mm for dimensions up to 100 mm and ÷2mm to÷4 mm for dimen-
sions greater than 100 mm. For Class 2, the tolerance for dimensions up to 100 mm
is ±1mm and ±1.5mm for dimensions over 100mm. Structural design to BS 5268 allows
for these tolerances and therefore analysis should be carried out for a `target' section. It is
the dimensions of the target section which should be included in specifications and on
drawings.
Timber and Plywood 119
Laminated timber products
Plywood
Plywood consists of veneers bonded together so that adjacent plies have the grain
running in orthogonal directions. Plywoods in the UK generally come from America,
Canada, Russia, Finland or the Far East, although the Russian and Far East plywood is
not listed in BS 5268 and therefore is not proven for structural applications. The type of
plywood available is dependent on the import market. It is worthwhile calling around
importers and stockists if a large or special supply is required. UK sizes are based on the
imperial standard size of 8
/
×4
/
(2.440 ×1.220 m). The main sources of imported ply-
wood in the UK are:
Canada and America The face veneer generally runs parallel to the longer side. Mainly
imported as Douglas fir 18 mm ply used for concrete shuttering, although 9 and 12mm are
also available. Considered a specialist structural product by importers.
Finland The face veneer can be parallel to the short or long side. Frequently spruce,
birch or birch faced ply. Birch plys are generally for fair faced applications, while spruce 9,
12, 18 and 24 mm thick is for general building use, such as flooring and roofing.
Glue laminated timber
Timber layers, normally 43 mm thick, are glued together to build up deep beam sections.
Long sections can be produced by staggering finger joints in the layers. Standard beam
widths vary from 90 mm to 240 mm although widths up to 265 mm and 290 mm are
available. Beam heights and lengths are generally limited to 2050 mm and 31 m respec-
tively. Column sections are available with widths of 90±200mm and depths of
90±420mm. Tapered and curved sections can also be manufactured. Loads are generally
applied at 90
·
to the thickness of the layers.
120 Structural Engineer's Pocket Book
Laminated veneered lumber (LVL)
LVL is similar to plywood but is manufactured with 3 mm veneers in a continuous
production line to create panels 1.8m wide, up to 26 m in length. It is quite a new
product, with relatively few UK suppliers. Beam sections for long spans normally have all
their laminations running longitudinally, while smaller, panel products tend to have
about a fifth of the laminations cross bonded to improve lateral bending strength.
Finnforest produce Kerto-S LVL for beams and Kerto-Q LVL for panels. Standard sections
are as follows:
Depth/width
(mm)
Thickness of panel (mm)
27 33 39 45 51 57 63 69 75
200
. . . . . . . . .
225
. . . . . . . . .
260
. . . . . . . .
300
. . . . . . .
360
. . . . . .
400
. . . . .
450
. . . .
500
. . .
600
. .
Kerto type S/Q S/Q S/Q S/Q S/Q S/Q S/Q S/Q S
Source: Finnforest (2002).
Timber and Plywood 121
Durability and fire resistance
Durability
Durablity of timber depends on its resistance to fungal decay. Softwood is more prone to
weathering and fungal attack than hardwood. Some timbers (such as oak, sweet chest-
nut, western red cedar and Douglas fir) are thought to be acidic and may need to
be isolated from materials such as structural steelwork. The durability of timber products
(such as plywood, LVL and glulam) normally depends on the stability and water resistance
of the glue.
Weathering
On prolonged exposure to sunlight, wind and rain, external timbers gradually lose their
natural colours and turn grey. Repeated wetting and drying cycles raise the surface grain,
open up surface cracks and increase the risk of fungal attack, but weathering on its own
generally causes few structural problems.
Fungal attack
For growth in timber fungi need oxygen, a minimum moisture content of 20% and
temperatures between 20
·
C and 30
·
C. Kiln drying at temperatures over 40
·
C will gen-
erally kill fungi, but fungal growth can normally be stopped by reducing the moisture
content. Where structural damage has occurred, the affected timber should be cut away
and replaced by treated timber. The remaining timber can be chemically treated to limit
future problems. Two of the most common destructive fungi are:
`Dry rot' ± Serpula lacrimans Under damp conditions, white cotton wool strands form
over the surface of the timber. Under drier conditions, a grey-white layer forms over the
timber with occasional patches of yellow or lilac. Fruiting bodies are plate-like forms
which disperse red spores. As a result of an attack, the timber becomes dry and friable
(hence the name dry rot) and breaks up into cube-like pieces both along and across the
grain.
`Wet rot' ± Coniophora puteana Known as cellar fungus, this fungus is the most
common cause of timber decay in the UK. It requires high moisture contents of
40±50% which normally result from leaks or condensation. The decayed timber is dark
and cracked along the grain. The thin strands of fungus are brown or black, but the
green fruiting bodies are rarely seen in buildings. The decay can be hidden below the
timber surface.
122 Structural Engineer's Pocket Book
Insect attack
Insect attack on timber in the UK is limited to a small number of species and tends to be
less serious than fungal attack. The reverse is generally true in hotter climates. Insects do
not depend on damp conditions although some species prefer timber which has already
suffered from fungal attack. Treatment normally involves removal of timber and treat-
ment with pesticides. Some common insect pests in the UK are:
`Common furniture beetle' ± nobium punctatum This beetle is the most widespread. It
attacks hardwoods and softwoods, and can be responsible for structural damage in
severe cases. The brown beetle is 3±5mm long; leaves flight holes of approximately 2mm
in diameter between May and September, and is thought to be present in up to 20 per cent
of all buildings.
`Wood boring weevils' ± Pentarthum huttonii and Euophrym confine Wood boring
beetles attack timber previously softened by fungal decay. Pentarthum huttonii is the
most common of the weevils and produces damage similar in appearance to the common
furniture beetle. The beetles are 3±5 mm long and leave 1 mm diameter flight holes.
`Powder post beetle' ± Lyctus brunneus The powder post beetle attacks hardwoods,
particularly oak and ash, until the sapwood is consumed. The extended soaking of
vulnerable timbers in water can reduce the risk of attack but this is not normally commer-
cially viable. The 4mm reddish-brown beetle leaves flight holes of about 1.5mm diameter.
`Death watch beetle' ± Xestobium rufovillosum The death watch beetle characteristi-
cally attacks partly decayed hardwoods, particularly oak, and is therefore responsible for
considerable damage to old or historic buildings. The beetles typically make tapping
noises during their mating season between March and June. Damp conditions encourage
infestation. The brown beetle is approximately 8mm long and leaves a flight hole of 3mm
diameter.
`Longhorn beetle' ± Hylotrupes bajulus The house long horn beetle is a serious pest,
mainly present in parts of southern England. The beetle can infest and cause significant
structural damage to the sapwood of seasoned softwood. Affected timbers bulge where
tunnelling has occurred just below the surface caused by larvae that can be up to 35 mm
long. The flight holes of the black beetle are oval and up to 10 mm across.
Source: BRE Digests 299, 307 and 345. Reproduced with permission by Building
Research Establishment.
Timber and Plywood 123
Fire resistance
Timber is an organic material and is therefore combustible. As timber is heated, water is
driven off as vapour. By the time it reaches 230±250
·
C, the timber has started to break
down into charcoal, producing carbon monoxide and methane (which cause flaming).
The charcoal will continue to smoulder to carbon dioxide and ash. However, despite its
combustibility, large sections of timber can perform better in fire than the equivalent
sections of exposed steel or aluminium. Timber has a low thermal conductivity which is
further protected by the charred surface, preventing the interior of the section fromburning.
BS 5268: Part 4 details the predicted rates of charring for different woods which allows
them to be `fire engineered'. Most timbers in BS 5268 have accepted charring rates of
20 mm in 30 minutes and 40 mm in 60 minutes. The exceptions are western red cedar
which chars more quickly at 25 mm in 30 minutes and 50 mm in 60 minutes, and oak,
utile, teak, jarrah and greenheart which all char slower at 15 mm in 30 minutes and
30mmin60minutes. Linear extrapolationis permittedfor periods between15and90minutes.
124 Structural Engineer's Pocket Book
Preliminary sizing of timber elements
Typical span/depth ratios for softwoods
Description Typical depth (mm)
Domestic floor (50mm wide joists at 400mm c /c) L/24+25 to 50
Office floors (50mm wide joists at 400mm c /c) L/15
Rafters (50mm wide joists at 400mm c /c) L/24
Beams/purlins L/10 to 15
Independent posts Min. 100mm square
Triangular trusses L/5 to 8
Rectangular trusses L/10 to 15
Plywood stressed skin panels L/30 to 40
Connections which rely on fixings (rather than dead bearing) to transfer the load in and out
of the timber can often control member size and for preliminary sizing. Highly stressed
individual members should be kept at about 50 per cent capacity until the connections can
be designed in detail.
Plywood stress skin panels
Stress skin panels can be factory made using glue and screws or on site just using screws. The
screws tend to be at close centres to accommodate the high longitudinal shear stresses.
Plywood can be applied to the top, or top and bottom, of the internal softwood joists
(webs). The webs are spaced according to the width of the panel and the point loads that
the panel will need to carry. The spacing is normally about 600mmfor a UDL of 0.75kN/m
2
,
about 400mmfor a UDL of 1.5kN/m
2
or about 300mmfor a UDL of more than 1.5kN/m
2
.
The direction of the face grain of the plywood skin will depend on the type of plywood
chosen for the panel. The top ply skin will need to be about 9±12mm thick for a UDL of
0.75kN/m
2
or about 12±18mmthick for a UDL of 1.5kN/m
2
. The bottomskin, if required, is
usually 8±9mm. The panel design is normally controlled by deflection and for economy the
EI of the trial section should be about 4.4WL
2
.
Timber and Plywood 125
Domestic floor joist capacity chart
See the graph below for an indication of the load carrying capacity of various joist sizes in
grade C16 timber spaced at 400mm centres.
Span tables for solid timber tongued and grooved decking
UDL*
kN/m
2
Single span (m) for
decking thickness
Double span (m) for
decking thickness
38mm 50mm 63mm 75mm 38mm 50mm 63mm 75mm
1.0 2.2 3.0 3.8 4.7 3.0 3.9 5.2 6.3
1.5 1.9 2.6 3.3 4.0 2.6 3.4 4.5 5.4
2.0 1.7 2.3 3.0 3.6 2.4 3.1 4.0 4.9
2.5 1.6 2.2 2.8 3.4 2.2 2.9 3.7 4.6
3.0 1.5 2.1 2.6 3.2 2.1 2.7 3.5 4.3
* These loads limit the deflection to span/240 as the decking is not normally used with a ceiling.
7.0
6.0
5.0
4.0
T
o
t
a
l

K
1
4
0.95
0.90
0.85
0.80
0 200 400 600
Width or depth of member (mm)
800 1000 1200
K
7
K
14
132 Structural Engineer's Pocket Book
Effective length of compression members
End conditions L
e
/L
Restrained at both ends in position and in direction 0.7
Restrained at both ends in position and one end in
direction
0.85
Restrained at both ends in position but not in direction 1.0
Restrained at one end in position and in direction and at
the other end in direction but not in position
1.5
Restrained at one end in position and direction and free
at the other end.
2.0
Generally the slenderness should be less than 180 for members carrying compression, or
less than 250 where compression would only occur as a result of load reversal due to
wind loading.
Compression buckling factor K
12
The stress in compression members should be less than the grade stress for compression
parallel to the grain modified for service class, load sharing, duration of load and K
12
for
slenderness. The following graph of K
12
has been calculated on the basis of E
min
and s
c|
based on long-terms loads.
Source: BS 5268: Part 2: 2002.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
50 100
D
4
0
C
1
6
,

C
2
4

o
r

D
5
0
150
L/r
y
K
1
2
200 250
Timber and Plywood 133
Deflection and stiffness factor K
9
Generally the limit on deflection of timber structure is 0.003 ×span or height. If this
requirement is met, both the elastic and shear deflections are considered to be controlled.
In domestic situations the total deflection must also be less than 14 mm. E
mean
can be
used in load-sharing situations. Elsewhere E
min
should be used, modified by K
9
for
trimmer joists and lintels. Glulam can be pre-cambered to compensate for deflections.
Number of pieces of timber
making up the element
K
9
Softwoods Hardwoods
1 1.00 1.00
2 1.14 1.06
3 1.21 1.08
_4 1.24 1.10
Source: BS 5268: Part 2: 2002.
134 Structural Engineer's Pocket Book
Timber joints
The code deals with nailed, screwed, bolted, dowelled and glued joints. Joint positions
and fixing edge distances can control member sizes, as a result of the reduced timber
cross section at the joint positions. Joint slip (caused by fixings moving in pre-drilled holes)
can cause rotations which will have a considerable effect on overall deflections.
Nailed joints
The values given for nailed joints are for nails made from steel wire driven at right angles
to the grain. In hardwood, holes normally need to be pre-drilled not bigger than 0.8d
(where d is the fixing diameter).
Minimum nail spacings for timber to timber joints
The following nail spacings can be reduced for all softwoods (except Douglas fir) by
multiplying by 0.8. However, the minimum allowable edge distance should never be less
than 5d.
5d
10d
20d 20d
Permissible load for a nailed joint in service
classes 1 and 2
F
adm
=F ×K
50
×n × the number of shear planes
n=the total number of nails in the joint
K
50
=0.9 for more than 10 nails in a line parallel to the action of the load. For nails driven
into the end grain of the timber a further factor of 0.7 should be used. For pre-drilled
holes a factor of 1.15 applies
Timber and Plywood 135
Basic single shear loads for nails in timber to timber joints
Nail diameter
mm
Basic shear load kN
Softwoods (not pre-drilled) Hardwoods (pre-drilled)
Standard
penetration
mm
Strength class Minimum
penetration
mm
Strength class
C16 C24 D40 D50
3 36 0.305 0.325 24 0.464 0.514
4 48 0.493 0.539 32 0.779 0.863
5 60 0.711 0.755 40 1.154 1.278
6 72 0.961 1.021 48 1.594 1.765
Basic single shear loads for nails in timber to plywood joints
Nominal
plywood
thickness
mm
Nail
diameter
mm
Nail
length
mm
Basic shear load*
kN
Softwoods
(not pre-drilled)
Hardwoods
(pre-drilled)
C16 C24 D40 and D50
6 3 50 0.256 0.267 0.295
4 75 0.360 0.360 0.360
12 3 50 0.286 0.296 0.352
4 75 0.430 0.446 0.545
18 3 50 0.344 0.355 0.417
4 75 0.485 0.501 0.601
21 3 50 0.359 0.374 0.456
4 75 0.520 0.537 0.641
* Additional capacity can be achieved for joints into Finnish birch and birch-faced plywoods, see BS
5268: Part 2: 2002: Table 63.
Source: BS 5268: Part 2: 2002: Appendix G.
136 Structural Engineer's Pocket Book
Screwed joints
The values given for screwed joints are for screws which conform to BS 1210 in pre-drilled
holes. The holes should be drilled with a diameter equal to that of the screw shank (f) for
the part of the hole to contain the shank, reducing to a pilot hole (with a diameter of f/2)
for the threaded portion of the screw. Where the standard headside thickness is less than
the values in the table, the basic load must be reduced by the ratio: actual/standard
thickness. The headside thickness must be greater than 2f twice the shank diameter. The
following tables give values for UK screws rather than the European screws quoted in the
latest British Standard.
Minimum screw spacings
5d
3d
10d 10d 10d
Permissible load for a screwed joint for service
classes 1 and 2
F
adm
=F ×K
54
×n
n=the total number of screws in the joint
K
54
=0.9 for more than 10 of the same diameter screws in a line parallel to the action of
the load. For screws inserted into the end grain of the timber a further factor of 0.7
should be used.
Timber and Plywood 137
Basic single shear loads for screws in timber to timber joints
Screw Standard
headside
thickness
mm
Basic single shear
kN
Screw
reference
Shank
diameter
mm
Softwoods Hardwoods
C16 C24 D40 and D50
No. 6 3.45 12 0.271 0.303 0.396
No. 8 4.17 16 0.407 0.440 0.584
No. 10 4.88 22 0.526 0.568 0.747
No. 12 5.59 35 0.738 0.796 1.053
No. 14 6.30 38 0.585 0.927 1.227
No. 16 7.01 44 1.005 1.090 1.442
Basic single shear loads for screws in timber to plywood joints
Nominal
plywood
thickness
mm
Screw Basic single shear*
kN
Screw
reference
Shank
diameter
mm
Minimum
screw
length
mm
Softwoods Hardwoods
C16 C24 D40 D50
12 No. 6 3.45 38 0.215 0.242 0.284 0.320
No. 8 4.17 38 0.303 0.326 0.426 0.490
No. 10 4.88 38 0.356 0.384 0.511 0.592
No. 12 5.59 38 0.478 0.522 0.712 0.834
No. 14 6.30 44 0.551 0.603 0.830 0.926
18 No. 6 3.45 57 0.287 0.299 0.351 0.381
No. 8 4.17 57 0.387 0.399 0.486 0.542
No. 10 4.88 57 0.429 0.455 0.567 0.638
No. 12 5.59 57 0.546 0.586 0.755 0.863
No. 14 6.30 57 0.764 0.829 1.109 1.288
* Extra capacity can be achieved for joints into Finnish birch and birch-faced plywoods, see BS 5268: Part 2:
2002: Table 68.
Source: BS 5268: Part 2: 2002: Appendix G.
138 Structural Engineer's Pocket Book
Bolted joints
The values given for bolted joints are for black bolts which conform to BS EN 20898-1
with washers which conform to BS 4320. Bolt holes should not be drilled more than
2mm larger than the nominal bolt diameter. Washers should have a diameter or width of
three times the bolt diameter with a thickness of 0.25 times the bolt diameter and be
fitted under the head and nut of each bolt. At least one complete thread should protrude
from a tightened nut.
Minimum bolt spacings
Permissible load for a bolted joint for service
classes 1 and 2
F
adm
=F ×K
57
×n
n= the total number of bolts in the joint
K
57
=1 ÷ (3(n ÷1)),100 for less than 10 of the same diameter bolts in a line parallel to
the action of the load
K
57
=0.7 for more than 10 of the same diameter bolts in a line parallel to the action of
the load
K
57
=1.0 for all other loading cases where more than one bolt is used in a joint
If a steel plate of minimum thickness 0.3 times the bolt diameter (or 2.5 mm) is bolted to
the timber, the basic load can be multiplied by a factor of 1.25. Further improvements on
the loads in bolts can be made by using toothed connectors, but these require larger
spacings (hence fewer fixings) and correct installation can be difficult.
Loaded
edge
parallel
to grain
Loaded edge perpendicular to grain
7d 4d
5d 4d 5d 4d
4d
4d
4d
4d
1.5d
4d
4d
4d 4d
Minimum
1.5d
1.5d
4d
4d
4d
Timber and Plywood 139
Basic single shear loads for one grade 4.6 bolt
in a two member timber connection
Timber
grade
Minimum
member
thickness
(mm)
Basic single shear load for selected grade 4.6 bolt
diameters in a two member* timber connection (kN)
Direction of loading
Parallel to the grain Perpendicular to the grain
M8 M12 M16 M20 M8 M12 M16 M20
C16 47 1.22 1.80 2.30 2.73 1.13 1.56 1.91 2.19
72 1.46 2.68 3.52 4.19 1.39 2.39 2.93 3.36
97 1.46 3.13 4.63 5.64 1.39 2.79 3.94 4.52
C24 47 1.33 2.04 2.59 3.09 1.23 1.76 2.16 2.47
72 1.55 2.93 3.97 4.73 1.47 2.64 3.30 3.79
97 1.55 3.42 5.05 6.37 1.47 3.07 4.43 5.11
D40 47 1.83 3.08 3.92 4.67 1.83 3.08 3.92 4.67
72 1.91 4.02 5.98 7.16 1.91 4.02 5.98 7.16
97 1.91 4.21 6.93 9.32 1.91 4.21 6.93 9.32
D50 47 2.12 3.78 4.81 5.73 2.12 3.78 4.81 5.73
72 2.12 4.66 6.92 8.78 2.12 4.66 6.92 8.78
97 2.12 4.66 8.09 10.82 2.12 4.66 8.09 10.82
* Extra capacity for three member connections can be achieved, see BS 5268: Part 2: 2002: Tables 76, 77,
79 and 80.
Source: BS 5268: Part 2: 2002: Appendix G.
140 Structural Engineer's Pocket Book
7
Masonry
Masonry, brought to the UK by the Romans, became a popular method of construction as
the units could originally be lifted and placed with one hand. Masonry has orthotropic
material properties relating to the bed or perpend joints of the masonry units. The
compressive strength of the masonry depends on the strength of the masonry units
and on the mortar type. Masonry is good in compression and has limited flexural
strength. Where the flexural strength of masonry `parallel to the bed joints' can be
developed, the section is described as `uncracked'. A cracked section (e.g. due to a damp
proof course or a movement joint) relies on the dead weight of the masonry to resist
tensile stresses. The structure should be arranged to limit tension or buckling in slender
members, or crushing of stocky structures.
Summary of material properties
Clay bricks The wide range of clays in the UK result in a wide variety of available brick
strengths, colours and appearance. Bricks can be hand or factory made. Densities range
between 22.5 and 28 kN/m
3
. Clay bricks tend to expand due to water absorption.
Engineering bricks have low water absorption, high strength and good durability
properties (Class A strength >70 N/mm
2
; water absorption _4.5% by mass. Class B:
strength >50 N/mm
2
; water absorption _7.0% by mass).
Calcium silicate bricks Calcium silicates are low cost bricks made from sand and slaked
lime rarely used due to their tendency to shrink and crack. Densities range between 17
and 21 kN/m
3
.
Concrete blocks Cement bound blocks are available in densities ranging between 5
and 20 kN/m
3
. The lightest blocks are aerated; medium dense blocks contain slag, ash or
pumice aggregate; dense blocks contain dense natural aggregates. Blocks can shrink by
0.01±0.09%, but blocks with shrinkage rates of no more than 0.03% are preferable to
avoid the cracking of plaster and brittle finishes on the finished walls.
Stone masonry Stone as rubble construction, bedded blocks or as facing to brick or
blockwork is covered in BS 5390. Thin stone used as cladding or facing is covered by BS
8298.
Cement mortar Sand, dry hydrate of lime and cement are mixed with water to form
mortar. The cement cures on contact with water. It provides a bond strong enough that
the masonry can resist flexural tension, but structural movement will cause cracking.
Lime mortar Sand and non-hydraulic lime putty form a mortar, to which some cement
(or other pozzolanic material) can be added to speed up setting. The mortar needs air, and
warm, dry weather to set. Lime mortar is more flexible than cement mortar and therefore
can resist considerably more movement without visible cracking.
Typical unit strengths of masonry
Material
(relevant BS)
Class Water
absorption
Typical unit
compressive
strength
N/mm
2
Fired clay bricks
(BS 3921)
Engineering A <4.5% >70
Engineering B 7.0% >50
Facings (bricks selected
for appearance)
10±30% 10±50
Commons (Class 1 to 15) 20±24%
Class 1 7
Class 2 14
Class 3 20
Class 15 105
Flettons 15±25% 15±25
Stocks (bricks without
frogs)
20±40% 3±20
Calcium silicate
bricks (BS 187)
Classes 2 to 7 14±48.5
Concrete bricks
(BS 6073: Pt 1)
7±20
Concrete blocks
(BS 6073: Pt 1)
Dense solid 7, 10±30
Dense hollow 3.5, 7, 10
Lightweight 2.8, 3.5, 4, 7
Reconstituted
stone (BS 6457)
Dense solid Typically as dense
concrete blocks
Natural stone
(BS 5390 and
BS 8298)
Structural quality. Strength is dependent
on the type of stone, the quality, the
direction of the bed, the quarry location
15±100
142 Structural Engineer's Pocket Book
Geometry and arrangement
Brick and block sizes
The standard UK brick is 215 ×102 ×65 mm which gives a co-ordinating size of
225×112 ×75 mm. The standard UK block face size is 440 ×215mm in thicknesses
from 75 to 215mm, giving a co-ordinating size of 450 ×225mm. This equates to two
brick stretchers by three brick courses.
The Health and Safety Executive (HSE) require designers to specify blocks which weigh less
than 20 kg to try to reduce repetitive strain injuries in bricklayers. Medium dense and
dense blocks of 140 mm thick, or more, often exceed 20 kg. The HSE prefers designers to
specify half blocks (such as Tarmac Topcrete) rather than rely on special manual handling
(such as hoists) on site. In addition to this, the convenience and speed of block laying is
reduced as block weight increases.
Frog
215
440
65
102
215
Varies
Perpend
Stretcher
Masonry 143
Non-hydraulic lime mortar mixes for masonry
Mix constituents Approximate
proportions
by volume
Notes on general application
Lime putty:coarse
sand
2:5 Used where dry weather and no frost
are expected for several months
Pozzolanic*:lime
putty:coarse sand
1:2±3:2 Used where an initial mortar set is
required within a couple of days
* Pozzolanic material can be cement, fired china dust or ground granulated blast furnace slag (ggbfs).
The actual amount of lime putty used depends on the grading of the sand and the volume
of voids. Compressive strength values for non-hydraulic lime mortar masonry can be
approximated using the values for Class IV cement mortar. Due to the flexibility of non-
hydraulic lime mortar, thermal and moisture movements can generally be accommodated
by the masonry without cracking of the masonry elements or the use of movement joints.
This flexibility also means that resistance to lateral load relies on mass and dead load
rather than flexural strength. The accepted minimum thickness of walls with non-hydraulic
lime mortar is 215mm and therefore the use of lime mortar in standard single leaf cavity
walls is not appropriate.
Cement mortar mixes for masonry
Mortar
class
Type of mortar (proportions by volume) Compressive
strengths N/mm
2
Cement:
lime:sand
Masonry
cement:sand
Cement:sand
with plasticizer
Lab Site
Dry pack 1:0:3 ± ± ± ±
I 1::3 ± ± 16.0 11.0
II 1::4 to 4 1:2 to 3 1:3 to 4 6.5 4.5
III 1:1:5 to 6 1:4 to 5 1:5 to 6 3.6 2.5
IV 1:2:8 to 9 1:5 to 6 1:7 to 8 1.5 1.0
NOTES:
1. Mix proportions are given by volume. Where sand volumes are given as variable amounts, use the
larger volume for well-graded sand and the smaller volume for uniformly graded sand.
2. As the mortar strength increases, the flexiblity reduces and likelihood of cracking increases.
3. Cement:lime:sand mortar provides the best bond and rain resistance, while cement:sand and
plasticizer is more frost resistant.
Source: BS 5628: Part 1: 1992.
144 Structural Engineer's Pocket Book
Selected bond patterns
For strength, perpends should not be less than one quarter of a brick from those in an
adjacent course.
English
bond
Flemish
bond
English garden
wall bond
Flemish garden
wall bond
Stretcher bond
Masonry 145
Movement joints in masonry with cement-based mortar
Movement joints to limit the lengths of walls built in cement mortar are required to
minimize cracking due to deflection, differential settlement, temperature change and
shrinkage or expansion. In addition to long wall panels, movement joints are also required
at points of weakness, where stress concentrations might be expected to cause cracks
(such as at steps in height or thickness or at the positions of large chases). Typical
movement joint spacings are as follows:
Material Approximate horizontal joint
spacing
2
and
reason for provision
Typical
joint thickness
mm
Maximum
suggested panel
length:height
ratio
1
Clay bricks 12m for expansion 16 3:1
15±18m with bed joint reinforcement at
450mm c /c
22
18±20m with bed joint reinforcement at
225mm c /c
25
Calcium
silicate bricks
7.5±9m for shrinkage 10 3:1
Concrete
bricks
6m for shrinkage 10 2:1
Concrete
blocks
6±7m for shrinkage 10 2:1
15±18m with bed joint reinforcement at
450mm c /c
22
18±20m with bed joint reinforcement at
225mm c /c
25
Natural stone
cladding
6m for thermal movements 10 3:1
NOTES:
1. Consider bed joint reinforcement for ratios beyond the suggested maximum.
2. The horizontal joint spacing should be halved for joints which are spaced off corners.
Vertical joints are required in cavity walls every 9 m or three storeys for buildings over
12 m or four storeys. This vertical spacing can be increased if special precautions are taken
to limit the differential movements caused by the shrinkage of the internal block and the
expansion of the external brick. The joint is typically created by supporting the external
skin on a proprietary stainless steel shelf angle fixed back to the internal structure.
Normally 1 mm of joint width is allowed for each metre of masonry (with a minimum
of 10 mm) between the top of the masonry and underside of the shelf angle support.
146 Structural Engineer's Pocket Book
Durability and fire resistance
Durability
Durability of masonry relies on the selection of appropriate components detailed to
prevent water and weather penetration. Wet bricks can suffer from spalling as a result
of frost attack. Bricks of low porosity are required in positions where exposure to moisture
and freezing is likely. BS 5268: Part 3 gives guidance on recommended combinations of
masonry units and mortar for different exposure conditions as summarized:
Durability issues for selection of bricks and mortar
Application Minimum
strength of
masonry unit
Mortar class
1
Brick frost
resistance
2
and soluble
salt content
3
Internal walls
generally/external
walls above DPC
Any block/15N/mm
2
brick
III FL, FN, ML,
MN, OL or ON
External below
DPC/freestanding
walls/parapets
7N/mm
2
dense
block /20 N/mm
2
brick
III FL or FN
(ML or MN if
protected from
saturation)
Brick damp proof
courses in
buildings (BS 743)
Engineering
brick A
I FL or
Earth retaining walls 7N/mm
2
dense
block /30 N/mm
2
brick
I or II FL or FN
Planter boxes Engineering brick/
20N/mm
2
commons
I or II FL or FN
Sills and copings Selected
block /30 N/mm
2
brick
I FL or FN
Manholes
and
inspection
chambers
Surface
water
Engineering brick /
20N/mm
2
commons
I or II FL or FN
(ML or MN if
more than
150mm below
ground level)
Foul
drainage
Engineering
brick A
I or II
NOTES:
1. Sulphate resisting mortar is advised where soluble sulphates are expected from the ground, saturated
bricks or elsewhere.
2. F indicates that the bricks are frost resistant, M indicates moderate frost resistance and O indicates no
frost resistance.
3. N indicates that the bricks have normal soluble salt content and L indicates low soluble salt content.
4. Retaining walls and planter boxes should be waterproofed on their retaining faces to improve
durability and prevent staining.
Fire resistance
As masonry units have generally been fired during manufacture, their performance in fire
conditions is generally good. Perforated and cellular bricks have a lesser fire resistance than
solid units of the same thickness. The fire resistance of blocks is dependent on the grading
of the aggregate and cement content of the mix, but generally 100mm solid blocks will
provide a fire resistance of up to 2 hours if load bearing and 4 hours if non-load bearing.
Longer periods of fire resistance may require a thicker wall than is required for strength.
Specific product information should be obtained from masonry manufacturers.
Masonry 147
Preliminary sizing of masonry elements
Typical span/thickness ratios
Description Typical thickness
Freestanding/
cantilever
Element
supported
on two sides
Panel
supported
on four sides
Lateral loading
Solid wall with no
piers ± uncracked section
H/6±8.5
1
H/20 or L/20 H/22 or L/25
Solid wall with no
piers ± cracked section
H/4.5±6.4
1
H/10 or L/20 H/12 or L/25
External cavity wall
2
panel ± H/20 or L/30 H/22 or L/35
External cavity wall
2
panel
with bed joint reinforcement
± H/20 or L/35 H/22 or L/40
External diaphragm wall
panel
H/10 H/14 ±
Reinforced masonry
retaining wall (bars in pockets
in the walls)
H/10±15
Solid masonry retaining wall
(thickness at base)
H/2.5±4 ± ±
Vertical loading
Solid wall H/8 H/18±22 ±
Cavity wall H/11 H/5.5 ±
Masonry arch/vault ± L/20±30 L/30±60
Reinforced brick beam depth ± L/10±16 ±
NOTES:
1. Depends on the wind exposure of the wall.
2. The spans or distances between lateral restraints are L in the horizontal direction and H in the vertical
direction.
3. In cavity walls, the thickness is the sum of both leaves excluding the cavity width.
148 Structural Engineer's Pocket Book
Vertical load capacity wall charts
Vertical load capacity of selected walls less than 150mm thick
Vertical load capacity of selected solid walls greater than 150mmthick
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1 100 Block (10 N/mm
2
)
Effective height (m)
P
e
r
m
i
s
s
i
b
l
e

(
N
/
m
m
2
)
9
10
11
12
13
Masonry 149
Preliminary sizing of external cavity wall panels
The following approach can be considered for cavity wall panels in non-load bearing
construction up to about 3.5 m tall in buildings of up to four storeys high, in areas which
have many windbreaks. Without major openings, cavity wall panels can easily span up to
3.5 m if spanning horizontally, while panels supported on four sides can span up to about
4.5 m. Load bearing wall panels can be larger as the vertical loads pre-compress the
masonry and give it much more capacity to span vertically. Gable walls can be treated as
rectangular panels and their height taken as the average height of the wall.
Detailed calculations for masonry around openings can sometimes be avoided if:
1. The openings are completely framed by lateral restraints.
2. The total area of openings is less than the lesser of: 10% of the maximum panel area
(see the section on the design of external wall panels to BS 5628 under `Lateral load'
later in this chapter) or 25% of the actual wall area.
3. The opening is more than half its maximum dimension from any edge of the wall
panel (other than its base) and from any adjacent opening.
150 Structural Engineer's Pocket Book
Internal non-loadbearing masonry partition chart
Typical ultimate strength values for stone masonry
Crushing Tension Shear Bending
N/mm
2
N/mm
2
N/mm
2
N/mm
2
Basalt 8.5 8.6 4.3 ±
Chalk 1.1 ± ± ±
Granite 96.6 3.2 5.4 10.7
Limestone 53.7 2.7 4.3 6.4
Limestone soft 10.7 1.0 3.8 5.4
Marble 64.4 3.2 5.4 ±
Sandstone 53.7 1.1 3.2 5.4
Sandstone soft 21.5 0.5 1.1 2.1
Slate 85.8 1.1 3.2 5.4
The strength values listed above assume that the stone is of good average quality and that
the factor of safely commonly used was 10. While this seems sensible for tension, shear
and bending it does seem conservative for crushing strength. Better values can be
achieved on the basis of strength testing. These values can be used in preliminary design,
but where unknown stones or unusual uses are proposed, strength testing is advised. The
strength of stone varies between sources and samples, and also depends on the mortar
and the manner of construction. The British Stone website has listings of stone tests
carried out by the Building Research Establishment (BRE).
As compressive load can be accompanied by a shear stress of up to half the compressive
stress, shear stresses normally control the design of slender items such as walls and piers.
Safe wall and pier loads are generally obtained by assuming a safe working compressive
stress equal to twice the characteristic shear stress.
Sources: BS 5628: Part 3: 2001;
Howe, J.A. (1910).
End restraints
required
Outside these
areas the
walls are
unstable
Top + end restraint
required
Top restraint
required
Top or end
restraint
required
H
e
i
g
h
t

c
a
p
a
c
i
t
y
,
V
c
(
N
/
m
m
2
)
d = 250
4.0
%
Reinforced Concrete 187
Design of solid slabs
Solid slabs are supported on walls or beams.
With simple supports the applied moment is about M = wl
x
l
y
,24 allowing for bending in
two directions, where l
x
and l
y
can be different span lengths.
Design moments and shear forces for a one way spanning continuous
solid slab
End support/slab connection At first
interior
support
At middle
of interior
span
At interior
supports
Simple support Continuous
At outer
support
Near middle
of end span
At outer
support
Near middle
of end span
Moment 0 WL
11.5
÷WL
25
WL
13
÷WL
11.5
WL
15.5
÷WL
15.5
Shear
W
2.5
±
6W
13
±
3W
5
±
W
2
Where W is the load on one span and L is the length of one span.
Design moments for a two way spanning continuous solid slab
Where l
y
,l
x
_ 1.5 the following formulae and coefficients can be used to calculate
moments in orthogonal directions M
x
=b
x
Wl
x
and M
y
=b
y
Wl
y
for the given edge conditions:
Type of panel Moments
considered*
Coefficient b
x
for short span l
x
l
y
l
x
= 1.0
l
y
l
x
= 1.2
l
y
l
x
= 1.5
Interior panel Continuous
edge
÷1
32
÷1
23
÷1
19
÷1
31
Midspan
1
41
1
29
1
25
1
41
One short edge
discontinuous
Continuous
edge
÷1
26
÷1
20
÷1
17
÷1
27
Midspan
1
34
1
26
1
23
1
35
One long edge
discontinuous
Continuous
edge
÷1
26
÷1
17
÷1
13
÷1
27
Midspan
1
33
1
22
1
18
1
35
Two adjacent
edges
discontinuous
Continuous
edge
÷1
21
÷1
15
÷1
12
÷1
22
Midspan
1
27
1
20
1
17
1
29
* These moments apply to the full width of the slab in each direction. The area of reinforcement to be provided top and bottom, both ways,
at corners where the slab is not continuous =75% of the reinforcement for the short span, across a width l
x
,5 both ways.
Form and area of shear reinforcement in solid slabs
The allowable shear stress, v
c
, is the same as that calculated for beams, but the slab
section should be sized to avoid shear reinforcement. If required, Table 3.16 in BS 8110
sets out the reinforcement requirements.
Source: BS 8110: Part 1: 1997.
Coefficient b
y
for long span l
y
188 Structural Engineer's Pocket Book
Design of flat slabs
Flat slabs are solid slabs on concrete which sit on points or columns instead of linear wall
or beam supports. Slab depth should be selected to satisfy deflection requirements and to
resist shear around the column supports. Any recognized method of elastic analysis can
be used, but BS 8110 suggests that the slabs be split into bay-wide subframes with
columns or sections of columns projecting above and below the slab.
Simplified bending moment analysis in flat slabs
A simplified approach is permitted by BS 8110 which allows moments to be calculated on
the basis of the values for one way spanning solid slabs on continuous supports less the
value of 0.15Wh
c
where h
c
=

4A
col
,p
_
and A
col
=column area. Alternatively, the follow-
ing preliminary moments for regular grid with a minimum of three bays can be used for
feasibility or preliminary design purposes only:
Preliminary target moments and forces for flat slab design
End support/slab
connection
At first
interior
support
At middle
of interior
span
At interior
supports
Simple support Continuous
At outer
support
Near
middle
of end span
At outer
support
Near
middle
of end
span
Column
strip
moments
0 WL
2
11
÷WL
2
20
WL
2
10
÷2WL
2
13
WL
2
11
÷2WL
2
15
Middle
strip
moments
0 WL
2
11
÷WL
2
20
WL
2
10
÷WL
2
20
WL
2
11
÷WL
2
20
W is a UDL in kN/m
2
, L is the length of one span and M is in kNm/m width of slab.
Moment transfer between the slab and exterior columns is limited to M
t max.
=0.15F
cu
b
e
d
2
where b
e
depends on the slab to column connection as given in Figure 3.13 in BS 8110.
Subframe moments may need to be adjusted to keep the assumed moment transfer
within the value of M
t max
.
Distribution of bending moments in flat slabs
The subframes used in the analysis are further split into middle and column strips. Loads
are more concentrated on the column strips. Typically, for hogging (negative) moments,
75% of the total subframe design moment will be distributed to the column strip. For
sagging (positive) moments, 55% of the total subframe design moment will be distributed
to the column strip. Special provision must be made for holes in panels and panels with
marginal beams or supporting walls. BS 8110 suggests that where l
y
,l
x
_ 2.0, column
strips are normally l
x
,2 wide centred on the grid. The slab should be detailed so that 66 per
cent of the support reinforcement is located in the width l
x
,4 centred over the column.
Reinforced Concrete 189
Punching shear forces in flat slabs
The critical shear case for flat slabs is punching shear around the column heads. The basic
shear, V, is equal to the full design load over the area supported by the column which
must be converted to effective shear forces to account for moment transfer between the
slab and columns.
For slabs with equal spans, the effective shears are: V
eff
=1.15V for internal columns,
V
eff
=1.25V for corner columns and V
eff
=1.25V for edge columns for moments parallel
to the slab edge or V
eff
=1.4V edge columns for moments perpendicular to the slab edge.
Punching shear checks in flat slabs
The shear stress at the column face should be checked: i
o
= V
eff
,U
o
d (where U
o
is the
column perimeter in contact with the slab). This should be less than the lesser of 0.8

d
_
where d is the
elastic deflection (in mm) under dead and permanent loads. In most cases problems due
to vibrations can be avoided as the natural frequency of the floor is kept greater than
4Hz±4.5Hz.
Composite Steel and Concrete 283
11
Structural Glass
Structural glass assemblies are those in which the self-weight of the glass, wind and other
imposed loads are carried by the glass itself rather than by glazing bars, mullions or transoms,
and the glass elements are connected together by mechanical connections or by adhesives.
Despite the increasing use of glass as a structural material over the last 25 years, there is
no single code of practice which covers all of the issues relating to structural glass
assemblies. Therefore values for structural design must be based on first principles,
research, experience and load testing. The design values given in this chapter should be
used very carefully with this in mind.
The following issues should be considered:
.
Glass is classed as a rigid liquid as its intermolecular bonds are randomly arranged,
rather than the crystalline arrangement normally associated with solids. Glass will
behave elastically until the applied stresses reach the level where the interatomic bonds
break. If sufficient stress is applied, these cracks will propagate and catastrophic failure
will occur. The random arrangement of the interatomic bonds means that glass is not
ductile and therefore failure is sudden.
.
Cracks in glass propagate faster as temperature increases.
.
Without the ability to yield, or behave plastically, glass can fail due to local over-
stressing. Steel can redistribute high local stresses by local yielding and small plastic
deformations, but glass cannot behave like this and high local stresses will result in
brittle failure.
.
Modern glass is not thought to deform or creep under long-term stresses. It behaves
perfectly elastically and will return to its original shape when applied loads are
removed. However, some old glass has been found to creep.
.
Glass will generally fail as a result of the build-up of tensile stresses. Generally it is the
outer surfaces of the glass which are subject to these stresses. Small flaws on glass
surfaces encourage crack propagation which can lead to failure. Structural glass should
be carefully checked for flaws.
.
Annealed glass can also fail as a result of `static fatigue'. There are various theories on why
this occurs, but in simple terms, microcracks form and propagate under sustained loads
resulting in failure of the glass. This means that the strength of glass is time dependent; in
the short term glass can carry about twice the load that it can carry in the long term.
Long-term stresses are kept low in design to prevent propagation of cracks. There is a
finite time for static fatigue failure to occur for each type of glass and although this is
beyond the scope of this book, this period can be calculated. It is about 15 days for
borosilicate glass (better known as Pyrex), but is generally much longer for annealed glass.
.
Thermal shock must also be considered for annealed glass. Temperature differences
across a single sheet of glass can result in internal stresses. Glass elements which are
partly in direct sunlight and partly shaded are at most risk of failure. Thermal shock
cracks tend to start at the edge of the glass, travelling inwards at about 90
·
, but this
type of failure can depend on many things including edge restraint, and manufacturers
should be consulted for each situation. If thermal shock is expected to be an issue,
toughened or tempered glass should be specified in place of annealed.
.
Glass must come from a known and reliable source to provide reliable strength and
minimal impurities.
Types of glass products
Annealed/float glass
Glass typically consists of frit: sand (silica 72%), soda ash (sodium carbonate 13%),
limestone (calcium carbonate 10%) and dolomite (calcium magnesium carbonate 4%).
This mixture is combined with broken glass (called cullet) at about 80% frit to 20% cullet,
and is heated to 1500
·
C to melt it. It leaves the first furnace at about 1050
·
C and goes on
to the forming process.
There are a number of forming processes, but structural glass is generally produced by the float
glass method, which was developed in 1959 by Pilkington. The molten glass flows out of the
furnace on to a bed of molten tin in a controlled atmosphere of nitrogen and oxygen and is
kept at high temperature. This means that defects and distortions are melted out of the upper
and lower surfaces without grinding and polishing. The glass is progressively cooled as it is
moved along the bath by rollers until it reaches about 600
·
C when the glass sheet becomes
viscous enough to come into contact with the rollers without causing damage to the bottom
surface. The speed at which the ribbon of glass moves along the tin bath determines the
thickness of the glass sheet. Finally the glass is cooled in a gradual and controlled manner to
200
·
Cin the annealing bay. The term`annealed' means that the glass has been cooled carefully
to prevent the build-up of residual stresses. The surfaces of float glass can be described by using
the descriptions `tin side' and `air side' depending on which way the glass was lying in the float
bath. For use as structural glass, the material should be free of impurities and discoloration. At
failure, annealed glass typically breaks into large pieces with sharp edges. Annealed glass must
therefore be specified carefully so that on failure it will not cause injury.
Toughened/fully tempered glass
Sheets of annealed glass are reheated to their softening point at about 700
·
C before
being rapidly cooled. This can be done by hanging the glass vertically gripped by tongs
with cooling applied by air jets, or by rolling the glass through the furnace and cooling
areas. The rapid cooling of the glass causes the outer surfaces to contract quicker than the
inner core. This means that a permanent precompression is applied to the glass, which
can make its capacity for tensile stress three to four times better than annealed glass. The
strength of toughened glass can depend on its country of origin. The values quoted in this
book relate to typical UK strengths. The `tin side' of toughened glass can be examined
using polarizing filters to determine the residual stresses and hence the strength of the
glass. Toughened glass cannot be cut or drilled after toughening, therefore glass is generally
toughened for specific projects rather than being kept as a stock item. Toughened glass is
more resistant to temperature differentials within elements than annealed glass and
therefore it tends to be used externally in elements such as floors where annealed glass
would normally be used in internal situations. Toughened glass can fail as a result of
nickel sulphide impurities as described in the section on `Heat soaked glass'. If specified to
BS 6206, toughened glass is regarded as a safety glass because it fractures into small
cubes (40 particles per 50 mm square) without sharp edges when broken. However, these
cubes have the same density as crushed concrete and the design should prevent broken
glass falling out of place to avoid injury to the public.
Partly toughened/heat tempered glass
Sheets of annealed glass are heated and then cooled in the same way as the toughening
process; however, the cooling is not as rapid. This means that slightly less permanent
precompression is applied to the glass, which will make its capacity for tensile stress 1.5
to 2 times better than annealed glass. The residual strength can be specified depending on
the proposed use. Heat tempered glass will not fail as a result of nickel sulphide impurities as
described for toughened glass in the section on `Heat soaked glass'. The fracture pattern is
very like that of annealed glass, as the residual stresses are not quite enough to shatter the
glass into the small cubes normally associated with toughened glass. Tempered glass can be
laminated to the top of toughened glass to produce units resistant to thermal shock, which
can remain in place with a curved shape even when both sheets of glass have been broken.
Structural Glass 285
Heat soaked glass
The float glass process leaves invisible nickel sulphide (NS) impurities in the glass called
inclusions. When the glass is toughened, the heat causes these inclusions to become
smaller and unstable. After toughening, and often after installation, thermal movements
and humidity changes can cause the NS inclusions in the glass to revert to their original
form by expanding. This expansion causes the glass to shatter and failure can be quite
explosive. Heat soaking can be specified to reduce the risk of NS failure of toughened
glass by accelerating the natural phenomenon. This accelerated fatigue will tend to break
flawed glass during a period of prolonged heating at about 300
·
C. Heat soaking periods
are the subject of international debate and range between 2 and 8 hours. The German
DIN standard is considered the most reliable code of practice. Glass manufacturers
indicate that one incidence of NS failure is expected in every 4 tonnes of toughened
glass, but after heat soaking this is thought to reduce to about 1 in 400 tonnes.
Laminated glass
Two or more sheets of glass are bonded, or laminated, together using plastic sheet or
liquid resin interlayers. The interlayer is normally polyvinyl butyral (PVB) built up in sheets
of 0.38 mm and can be clear or tinted. It is normal to use four of these layers (about
12 mm) in order to allow for the glass surface ripple which is produced by the rollers used
in the float glass manufacturing process. Plastic sheets are used for larger numbers and
sizes of panels. Liquid resins are more suited to curved glass and to small-scale manu-
facturing, as the glass sheets have to be kept spaced apart in order to obtain a uniform
thickness of resin between the sheets. The bonding is achieved by applying heat and
pressure in an autoclave. If the glass breaks in service, the interlayer tends to hold the
fragmented glass to the remaining sheet until the panel can be replaced. Laminates can
be used for safety, bullet proofing, solar control and acoustic control glazing. Toughened,
tempered, heat soaked and annealed glass sheets can be incorporated and combined in
laminated panels. Ideally the glass should be specified so that toughened or tempered
glass is laminated with the `tin side' of the glass outermost, so that the glass strength can
be inspected if necessary. Laminated panels tend to behave monolithically for short time
loading at temperatures below 70
·
C, but interlayer creep means that the layers act
separately under long-term loads.
Summary of material properties
Density 25±26 kN/m
3
Compressive strength F
cu
=1000 N/mm
2
Tensile strength Strength depends on many factors including: duration of loading,
rate of loading, country of manufacture, residual stresses, temperature, size of cross
section, surface finish and micro cracks. Fine glass fibres have tensile strengths of up to
1500 N/mm
2
but for the sections used in structural glazing, typical characteristic tensile
strengths are: 45 N/mm
2
for annealed, 80 N/mm
2
for tempered glass and 120 N/mm
2
for
toughened. Patterned or wired glass can carry less load.
Modulus of elasticity 70±74 kNmm
2
Poisson's ratio 0.22±0.25
Linear coefficient of
thermal expansion 8 ×10
÷6
/
·
C
286 Structural Engineer's Pocket Book
Typical glass section sizes and thicknesses
The range of available glass section sizes changes as regularly as the plant and facilities in the
glass factories are updated or renewed. There are no standard sizes, only maximum sizes.
Manufacturers should be contacted for up to date information about the sheet sizes available.
The contact details for Pilkington, Solaglass Saint Gobain, Firman, Hansen Glass, QTG,
European, Bishoff Glastechnik and Eckelt are listed in the chapter on useful addresses. Always
check that the required sheet size can be obtained and installed economically.
Annealed/float glass
The typical maximum size is 3210mm×6000mm although sheets up to
3210mm×8000mm can be obtained on special order or from continental glass manu-
facturers.
Typical float glass thicknesses
Thickness mm 3* 4 5* 6 8* 10 12 15 19 25
Weight kg/m
2
7.5 10.0 12.5 15.0 20.0 25.0 30.0 37.5 47.5 62.5
* Generally used in structural glazing laminated units.
Toughened/fully tempered glass
The sizes of toughened sheets are generally smaller than the sizes of float glass available.
Toughened glass in 25mm is currently still only experimental and is generally not available.
Thickness
mm
4 5 6 8 10 12 15 19
Sheet size*
mm×mm
1500×
2200
2000 ×
4200
2000×
4200
2000 ×
4200
2000×
4200
2000×
4200
1700×
4200
1500 ×
4200
* Larger sizes are available from certain UK and European suppliers.
Heat tempered/partly toughened glass
Normally only produced in 8 mm thick sheets for laminated units. Manufacturers should
be consulted about the availability of 10 mm and 12 mm sheets. Sheet sizes are the same
as those for fully toughened glass.
Heat soaked glass
Sheet sizes are limited to the size of the heat soaking oven, typically about
2000mm×6000 mm.
Laminated glass
Limited only by the size of sheets available for the different types of glass and the size of
autoclave used to cure the interlayers.
Curved glass
Curved glass can be obtained in the UK fromPilkington with a minimumradius of 750mm for
12mm glass; a minimum radius of 1000mm for 15mm glass and a minimum radius of
1500mm for 19mm glass. However, Sunglass in Italy and Cricursa in Spain are specialist
providers who can provide a minimum radius of 300mm for 10mm glass down to 100mm
for 4mm to 6mm glass.
Structural Glass 287
Durability and fire resistance
Durability
Glass and stainless steel components are inherently durable if they are properly specified
and kept clean. Glass is corrosion resistant to most substances apart from strong alkalis.
The torque of fixing bolts and the adhesives used to secure them should be checked
approximately every 5 years and silicone joints may have to be replaced after 25±30 years
depending on the exposure conditions. Deflection limits might need to be increased to
prevent water ingress caused by rotations at the framing and sealing to the glass.
Fire resistance
Fire resistant glasses are capable of achieving 60 minutes of stability and integrity when
specially framed using intumescent seals etc. There are several types of fire resisting glass
which all have differing amounts of fire resistance. The wire interlayer in Georgian wired
glass maintains the integrity of the pane by holding the glass in place as it is softened by
the heat of a fire. Intumescent interlayers in laminated glass expand to form an opaque
rigid barrier to contain heat and smoke. Prestressed borosilicate glass (better known as
Pyrex) can resist heat without cracking but must be specially made to order and is limited
to 1.2 m by 2 m panels.
288 Structural Engineer's Pocket Book
Typical glass sizes for common applications
The following are typical sizes from Pilkington for standard glass applications. The normal
design principles of determinacy and redundancy should also be considered when using
these typical sizes. These designs are for internal use only. External use requires more
careful consideration of thermal effects, where it may be more appropriate to specify
toughened glass instead of annealed glass.
Toughened glass barriers
Horizontal line load
kN/m
Toughened glass thickness*
mm
0.36 12
0.74 15
1.50 19
3.00 25
* For 1.1 m high barrier, clamped at foot.
Toughened glass infill to barriers bolted between uprights
Loading Limiting glass span for glass thickness
m
UDL Point load 6mm* 8mm* 10mm 12mm
kN/m
2
kN
0.5 0.25 1.40 1.75 2.10 2.40
1.0 0.50 0.90 1.45 1.75 2.05
1.5 1.50 ± ± 1.20 1.60
* Not suitable if free path beside barrier is >1.5m as it will not contain impact loads as Class A to BS
6206.
Laminated glass floors and stair treads
UDL
kN/m
2
Point load
kN
Glass thickness
(top÷bottom annealed)*
Typical use
mm÷mm
1.5 1.4 19÷10 Domestic floor or stair
5.0 3.6 25÷15 Dance floor
4.0 4.5 25÷25 Corridors
4.0 4.0 25÷10 Stair tread
* Based on a floor sheet size of 1m
2
or a stair tread of 0.3×1.5 m supported on four edges with a
minimum bearing length equal to the thickness of the glass unit. The 1m
2
is normally considered to be
the maximum size/weight which can be practically handled on site.
Structural Glass 289
Glass mullions or fins in toughened safety glass
Mullion height
m
Mullion thickness/depth for wind loading
mm*
1.00kN/m
2
1.25kN/m
2
1.50kN/m
2
1.75kN/m
2
<2.0 19/120 19/130 25/120 25/130
2.0±2.5 19/160 25/160 25/170 ±
2.5±3.0 25/180 25/200 ± ±
3.0±3.5 25/230 ± ± ±
3.5±4.0 25/280 ± ± ±
* Assuming restraint at head and foot plus sealant to main panels.
Glass walls and planar glazing
Suspended structural glass walls can typically be up to 23 m high and of unlimited length,
while ground supported walls are usually limited to a maximum height of 9 m.
Planar glazing is limited to a height of 18 m with glass sheet sizes of less than 2m
2
so that
the weights do not exceed the shear capacity of the planar bolts and fixings.
In a sheltered urban area, 2 ×2 m square panels will typically need a bolt at each of the
four corners; 2 ×3.5 m panels will need six bolts and panels taller than 3.5 m will need
eight bolts.
Source: Pilkington (2002).
290 Structural Engineer's Pocket Book
Structural glass design
Summary of design principles
.
Provide alternative routes within a building so that users can choose to avoid crossing
glass structures.
.
Glass is perfectly elastic, but failure is sudden.
.
Deflection and buckling normally govern the design. Deflections of vertical panes are
thought to be acceptable to span/150, while deflections of horizontal elements should
be limited to span/360.
.
Glass works best in compression, although bearing often determines the thickness of
beams and fins.
.
For designs in pure tension, the supports should be designed to distribute the stresses
uniformly across the whole glass area.
.
Glass can carry bending both in and out of its own plane.
.
Use glass in combination with steel or other metals to carry tensile and bending
stresses.
.
The sizes of glass elements in external walls can be dictated by energy efficiency
regulations as much as the required strength.
.
Keep the arrangement of supports simple, ensuring that the glass only carries pre-
dictable loads to avoid failure as a result of stress concentrations. Isolate glass from
shock and fluctuating loads with spring and damped connections.
.
Sudden failure of the glass elements must be allowed for in the design by provision of
redundancy, alternative load paths, and by using the higher short-term load capacity of
glass.
.
Glass failure should not result in a disproportionate collapse of the structure.
.
Generally long-term stresses in annealed glass are kept low to prevent failure as a result of
static fatigue, i.e. time dependent failure. For complex structures with simple loading condi-
tions, it is possible to stress glass elements for a calculated failure period in order to promote
failure by static fatigue.
.
The effects of failure and the method of repair or replacement must be considered in
the design, as well as maintenance and access issues.
.
The impact resistance of each element should be considered to establish an appropriate
behaviour as a result of damage by accident or vandalism.
.
It is good practice to laminate glass sheets used overhead. Sand blasting, etching and
fritting can be used to provide slip resistance and modesty for glass to be used under-
foot.
.
Toughened glass elements should generally be heat soaked to avoid nickel sulphide
failure if they are to be used to carry load as single or unlaminated sheets.
.
Consider proof testing elements/components if the design is new or unusual, or where
critical elements rely on the additional strength of single ply toughened glass.
.
Glass sheet sizes are limited to the standard sizes produced by the manufacturers and
the size of sheets which cutting equipment can handle.
.
When considering large sheet sizes, thought must be given to the practicalities of weight,
method of delivery and installation and possible future replacement.
.
Inspect glass delivered to site for damage or flaws which might cause failure.
.
Check that the glass can be obtained economically, in the time available.
Structural Glass 291
Codes of practice and design standards
There is no single code of practice to cover structural glass, although the draft Eurocode
pr EN 13474 `Glass in Building' is the nearest to an appropriate code of practice for
glass design. It is thought to be slightly conservative to account for the varying quality
of glass manufacture coming from different European countries. Other useful references
are BS 6262, Building Regulations Part N, Glass and Glazing Federation Data Sheets,
Pilkington Design Guidance Sheets; the IStructE Guide to Structural Use of Glass in
Buildings and the Australian standard AS 1288.
Glass can carry load in compression, tension, bending, torsion and shear, but the
engineer must decide how the stresses in the glass are to be calculated, what levels of
stress are acceptable, what factor of safety is appropriate and how can unexpected or
changeable loads be avoided. Overdesign will not guarantee safety.
Although some design methods use fracture mechanics or Weibull probabilities, the
simplest and most commonly used design approach is elastic analysis.
Guideline allowable stresses
The following values are for preliminary design using elastic analysis with unfactored
loads and are based on the values available in pr EN 13474.
Glass type Characteristic
bending
strength
Loading
condition
Typical factor
of safety
Typical
allowable
bending stress
N/mm
2
N/mm
2
Annealed 45 Long term 6.5 7
Short term 2.5 20
Heat tempered 70 Long term 3.5 20
Short term 2.4 30
Toughened 120 Long term 3.0 40
Short term 2.4 50
292 Structural Engineer's Pocket Book
Connections
Connections must transfer the load in and out of glass elements in a predictable way
avoiding any stress concentrations. Clamped and friction grip connections are the most
commonly used for single sheets. Glass surfaces are never perfectly smooth and connections
should be designed to account for differences of up to 1mm in the glass thickness. Cut
edges can have tolerances of 0.2±0.3mm if cut with a CNC laser, otherwise dimension
tolerances can be 1.0±1.5mm.
Simple supports
The sheets of glass should sit perfectly on to the supports, either in the plane, or
perpendicular to the plane, of the glass. Gaskets can cause stress concentrations and
should not be used to compensate for excessive deviation between the glass and the
supports. The allowable bearing stress is generally limited to about 0.42±1.5 N/mm
2
depending on the glass and setting blocks used.
Friction grip connections
Friction connections use patch plates to clamp the glass in place and are commonly used
for single ply sheets of toughened glass. More complex clamped connections can use
galvanized fibre gaskets and holes lined with nylon bushes to prevent stress concentrations.
Friction grip bolt torques should be designed to generate a frictional clamping force of
N=F/m, where the coefficient of friction is generally m=0.2.
Holes
Annealed glass can be drilled. Toughened or tempered glass must be machined before
toughening. The Glass and Glazing Federation suggest that the minimum clear
edge distance should be the greater of 30 mm or 1.5 times the glass thickness (t). The
minimum clear corner distance and minimum clear bolt spacing should be 4t. Holes
should be positioned in low stress areas, should be accurately drilled and the hole
diameter must not be less than the glass thickness.
Bolted connections
Bolted connections can be designed to resist loads, both in and out of the plane of the
glass. Pure bolted connections need to be designed for strength, tolerance, deflection,
thermal and blast effects. They can be affected by minor details (such as drilling accuracy
or the hole lining/bush) and this is why proprietary bolted systems are most commonly
used. Extensive testing should be carried out where bolted connections are to be
specially developed for a project.
Structural Glass 293
Non silicon adhesives
The use of adhesives (other than silicon) is still fairly experimental and as yet is generally
limited to small glass elements. Epoxies and UV cure adhesives are among those which
have been tried. It is thought that failure strengths might be about ten times those of
silicon, but suitable factors of safety have not been made widely available. Loctite and 3M
have some adhesive products which might be worth investigating/testing.
Structural silicones
Sealant manufacturers should be contacted for assistance with specifying their silicon
products. This assistance can include information on product selection, adhesion, com-
patibility, thermal/creep effects and calculation of joint sizes. Data from one project
cannot automatically be used for other applications.
Structural silicon sealant joints should normally be a minimum of 6mm×6 mm, with a
maximum width to depth ratio of 3:1. If this maximum width to depth ratio is exceeded,
the glass sheets will be able to rotate causing additional stresses in the silicon. A simplified
design approach to joint rotation can be used (if the glass deflection is less than L /100)
where reduced design stresses are used to allow additional capacity in the joint to cover
any rotational stresses. If joint rotation is specifically considered in the joint design
calculations, higher values of allowable design stresses can be used.
Dow Corning manufacture two silicon adhesives for structural applications. Dow Corning
895 is one part adhesive, site applied silicon used for small-scale remedial applications or
where a two-sided structurally bonded system has to be bonded on site. Dow Corning
993 is a two part adhesive, normally factory applied. The range of colours is limited and
availability should be checked for each product and application.
Technical data on the Dow Corning silicones is set out below:
Dow
Corning
silicon
Young's
modulus
kN/mm
2
Type of
stress
Failure
stress
N/mm
2
Loading
condition
Typical
allowable
design
stress
N/mm
2
933
(2 part)
0.0014 Tension/
compression
0.95 Short-term/live loads. Design
stress for comparison with
simplified calculations not
allowing for stresses due to
joint rotation
0.140
Short-term/live loads. Design
stress where the stresses due to
joint rotation for a particular
case have been specifically
calculated
0.210
Long-term/dead loads n/a
Shear 0.68 Short-term/live loads 0.105
Long-term/dead loads 0.011
895
(1 part)
0.0009 Tension/
compression
1.40 Short-term/live loads. Design
stress for comparison with
simplified calculations not
allowing for stresses due to
joint rotation
0.140
Short-term/live loads. Design
stress where the stresses due to
joint rotation for a particular
case have been specifically
calculated
0.210
Long-term/dead loads n/a
Shear 1.07 Short-term/live loads 0.140
Long-term/dead loads 0.007
Source: Dow Corning (2002).
294 Structural Engineer's Pocket Book
12
Building Elements, Materials,
Fixings and Fastenings
Waterproofing
Although normally detailed and specified by an architect, the waterproofing must co-
ordinate with the structure and the engineer must understand the implications of the
waterproofing on the structural design.
Damp proof course
A damp proof course (DPC) is normally installed at the top and bottom of external walls to
prevent the vertical passage of moisture through the wall. Cavity trays and weep holes are
required above the position of elements which bridge the cavity, such as windows or
doors, in order to direct any moisture in the cavity to the outside. The inclusion of a DPC
will normally reduce the flexural strength of the wall.
DPCs should:
.
Be bedded both sides in mortar to prevent damage.
.
Be lapped with damp proof membranes in the floor or roof.
.
Be lapped in order to ensure that moisture will flow over and not into the laps.
.
Not project into cavities where they might collect mortar and bridge the cavity.
Different materials are available to suit different situations:
.
Flexible plastic sheets or bitumen impregnated fabric can be used for most DPC
locations but can be torn if not well protected and the bituminous types can sometimes
be extruded under high loads or temperatures.
.
Semi-rigid sheets of copper or lead are expensive but are most effective for intricate
junctions.
.
Rigid DPCs are layers of slate or engineering brick in Class I mortar and are only used in
the base of retaining walls or freestanding walls. These combat rising damp and (unlike
the other DPC materials) can transfer tension through the DPC position.
Damp proof membrane
Damp proof membranes (DPMs) are sheet or liquid membranes which are typically
installed at roof and ground floor levels. In roofs they are intended to prevent the ingress
of rain and at ground floor level they are intended to prevent the passage of moisture
from below by capillary action. Sheet membranes can be polyethylene, bituminous or
rubber sheets, while liquid systems can be hot or cold bitumen or epoxy resin.
Basement waterproofing
Basement waterproofing is problematic as leaks are only normally discovered once the
structure has been occupied. The opportunity for remedial work is normally limited,
quite apart from the difficulty of reaching externally applied tanking systems. Although an
architect details the waterproofing for the rest of the building, sometimes the engineer is
asked to specify the waterproofing for the basement. In this case very careful co-ordination of
the lapping of the waterproofing above and below ground must be achieved to ensure that
there are no weak points. Basement waterproofing should always be considered as a three-
dimensional problem.
It is important to establish whether the system will be required to provide basic resistance
to water pressure, or whether special additional controls on water vapour will also be
required.
Basement waterproofing to BS 8102
BS 8102 sets out guidance for the waterproofing of basement structures according to
their use. The following table has been adapted from Table 1 in BS 8102: 1990 to include
some of the increased requirements suggested in CIRIA Report 139.
Methods of basement waterproofing
The following types of basement waterproofing systems can be used individually or
together depending on the building requirements:
Tanked This can be used internally or externally using painted or sheet membranes.
Externally it is difficult to apply and protect under building site conditions, while internally
water pressures can blow the waterproofing off the wall; however, it is often selected as it
is relatively cheap and takes up very little space.
Integral Concrete retaining walls can resist the ingress of water in differing amounts
depending on the thickness of the section, the applied stresses, the amount of reinforce-
ment and the density of the concrete. The density of the concrete is directly related to how
well the concrete is compacted during construction. Integral structural waterproofing
systems require a highly skilled workforce and strict site control. However, moisture and
water vapour can still pass through a plain wall and additional protection should be added if
this moisture will not be acceptable for the proposed basement use. BS 8007 provides
guidance on the design of concrete to resist the passage of water, but this still does not stop
water vapour. Alternatively, Caltite or Pudlo additives can be used with a BS 8110 structure
to create `waterproof' concrete. This is more expensive than standard concrete but this can
be offset against any saving on the labour and installation costs of traditional forms of
waterproofing.
Drained Drained cavity and floor systems allow moisture to penetrate the retaining
wall. The moisture is collected in a sump to be pumped away. Drained cavity systems tend
to be expensive to install and can take up quite a lot of basement floor area, but they are
thought to be much more reliable than other waterproofing systems. Draining ground
water to the public sewers may require a special licence from the local water authority.
Access hatches for the inspection and maintenance of internal gulleys should be provided
where possible.
296 Structural Engineer's Pocket Book
Basement waterproofing to BS 8102
Grade of
basement
to BS 8102
Basement
use
Performance
of water
proofing
Form of construction Comments
1 Car parking,
plant rooms (excluding
electrical equipment) and
workshops
Some water seepage and
damp patches tolerable
(typical relative humidity >65%)
Type B ± RC to BS 8110
(with crack widths limited to 0.3mm)
Provides integral protection and needs waterstops at construction joints.
Medium risk. Consider ground chemicals for durability and effect on
finishes. The BS 8102 description of a workshop is not as good as the
workshop environment described in the Building Regulations
2 Workshops and plantrooms
requiring drier environment.
Retail storage areas
No water penetration but moisture
tolerable (typical relative
humidity =35±50%)
Type A Requires drainage to external basement perimeter below the level of the
wall/floor membrane lap. Medium risk with multiple membrane layers and
strict site control
Type B ± RC to BS8007 Provides integral protection and needs waterstops at construction joints.
Medium risk. Consider ground chemicals for durability and effect on finishes.
Additional tanking is likely to be needed to meet retail storage requirements
3 Ventilated residential and
working areas, offices,
restaurants and leisure centres
Dry
environment, but no specific control
on moisture vapour (typical relative
humidity 40±60%)
Type A Not recommended unless drainage is provided above the wall/floor
membrane lap position and the site is relatively free draining. High risk
Type B ± RC to BS 8007 Provides integral protection and needs waterstops at construction joints.
Medium risk. Consider ground chemicals for durability and effect on
finishes. Additional tanking is recommended
Type C ± wall and floor cavity system A drained cavity allows the wall to leak and it is therefore foolproof.
Sumps may need back-up pumps. High safety factor
4 Archives and computer stores Totally dry environment with strict
control of moisture vapour (typical
relative humidity =35% for books ±
50% for art storage)
Type A Unikely to be able to provide the controlled conditions required. Very high
risk
Type B ± RC to BS 8007
plus vapour barrier
High risk. Medium risk with addition of a drainage cavity
to reduce water penetration
Type C ± wall
and floor cavity system with vapour barrier to
inner skin and floor cavity with DPM
Medium risk. Addition of a water resistant concrete wall would provide the
maximum possible safety for sensitive environments
NOTES:
1. Type A=tanked construction, Type B=integral structural waterproofing and Type C=drained protection.
2. Relative humidity indicates the amount of water vapour in the air as a percentage of the maximum amount of water vapour which would be possible for air at a given temperature and pressure. Typical values of relative humidity for the
UK are about 40±50% for heated indoor conditions and 85% for unheated external conditions.
Source: BS 8102: 1990.
2
9
7
Remedial work
Failed basement systems require remedial work. Application of internal tanking in this
situation is not normally successful. The junction of the wall and floor is normally the
position where water leaks are most noticeable.
An economical remedial method is to turn the existing floor construction into a drained
floor by chasing channels in the existing floor finishes around the perimeter. Additional
channels may cross the floor where there are large areas of open space. Proprietary plastic
trays with perforated sides and bases can be set into the chases, connected up and
drained to a sump and pump. New floor finishes can then be applied over the original
floor and its new drainage channels, to provide ground water protection with only a small
thickness of additional floor construction.
298 Structural Engineer's Pocket Book
Screeds
Screeds are generally specified by an architect as a finish to structural floors in order to
provide a level surface, to conceal service routes and/or as a preparation for application of
floor finishes. Historically screeds fail due to inadequate soundness, cracking and curling and
therefore, like waterproofing, it is useful for the engineer to have some background knowl-
edge. Structural toppings generally act as part of a precast structural floor to resist vertical
load or to enhance diaphragm action. The structural issues affecting the choice of screed are:
type of floor construction, deflection, thermal or moisture movements, surface accuracy and
moisture condition.
Deflection
Directly bondedscreeds canbe successfully appliedtosolidreinforcedconcrete slabs as they are
generally sufficiently rigid, while floating screeds are more suitable for flexible floors (such as
precast planks or composite metal decking) to avoid reflective cracking of the screed. Floating
screeds must be thicker than bonded screeds to withstand the applied floor loadings and are
laid on a slip membrane to ensure free movement and avoid reflective cracking.
Thermal/moisture effects
Drying shrinkage and temperature changes will result in movement in the structure, which
couldleadtothe crackingof an overlyingbonded screed. It is general practice to leave concrete
slabs to cure for 6 weeks before laying screed or applying rigid finishes such as tiles, stone or
terrazzo. For other finishes the required floor slab drying times vary. If movement is likely to be
problematic, joints should be made in the screed at predetermined points to allowexpansion/
contraction/stress relief.
Sand:cement screeds must be cured by close covering with polythene sheet for 7 days
while foot traffic is prevented and the screed is protected from frost. After this the
remaining free moisture in the screed needs time to escape before application of finishes.
This is especially true if the substructure and finish are both vapour proof as this can result
in moisture being trapped in the screed. Accurate prediction of screed drying times is
difficult, but a rough rule is 4 weeks per 25 mm of screed thickness (to reach about 75%
relative humidity). Accelerated heating to speed the drying process can cause the screed
to crack or curl, but dehumidifiers can be useful.
Surface accuracy
The accuracy of surface level and flatness of a laying surface is related to the type of base,
accuracy of the setting out and the quality of workmanship. These issues should be
considered when selecting the overall thickness of the floor finishes to avoid problems
with the finish and/or costly remedial measures.
Building Elements, Materials, Fixings and Fastenings 299
Precast concrete hollowcore slabs
The values for the hollowcore slabs set out below are for precast prestressed concrete slabs by
Tarmac Topfloor. The prestressing wires are stretched across long shutter beds before the
concrete is extruded or slip formed along beds up to 130m long. The prestress in the units
induces a precamber. The overall camber of associated units should not normally exceed L /300.
Some planks may need a concrete topping (not screed) to develop their full bending capacity or
to contribute to diaphragm action. Minimum bearing lengths of 100mm are required for
masonry supports, while 75mmis acceptable for supports on steelwork or concrete. Planks are
normally 1200mm wide at their underside and are butted up tight together on site. The units
are only 1180 to 1190mm wide at the top surface and the joints between the planks are
grouted up on site. Narrower planks are normally available on special order in a few specific
widths. Special details, notches, holes and fixings should be discussed with the plank manu-
facturer early in the design.
Typical spiroll hollowcore working load capacities
Nominal
hollowcore
plank depth
mm
Fire
resistance
hours
Typical
self-
weight
kN/m
2
Clear span for imposed loads*
m
1.5
kN/m
2
3
kN/m
2
5
kN/m
2
10
kN/m
2
150 up to 2 2.33 5.5±7.5 5.0±7.0 4.5±6.0 3.5±4.5
200 up to 2 2.94 7.5±10.0 7.0±8.5 6.0±7.5 4.5±6.0
260 2 3.97 10.0±12.0 8.5±11.0 7.5±10.0 6.0±8.0
320 2 3.97 12.0±14.5 11.0±13.0 10.0±11.5 8.0±9.5
400 2 4.83 14.5±17.5 13.0±15.5 11.5±14.0 9.5±11.5
*An allowance of 1.5 kN/m
2
for screeds and finishes has been included in addition to the
plank self-weight.
Source: Tarmac Topfloor (2002). Note that this information is subject to change at any
time. Consult the latest Tarmac literature for up to date information.
300 Structural Engineer's Pocket Book
Bi-metallic corrosion
When two dissimilar metals are put together with an `electrolyte' (normally water) an
electrical current passes between them. The further apart the metals are on the galvanic
series, the more pronounced this effect becomes.
The current consists of a flow of electrons from the anode (the metal higher in the
galvanic series) to the cathode, resulting in the `wearing away' of the anode. This effect
is used to advantage in galvanizing where the zinc coating slowly erodes, sacrificially
protecting the steelwork. Alloys of combined metals can produce mixed effects and
should be chosen with care for wet or corrosive situations in combination with other
metals.
The amount of corrosion is dictated by the relative contact surface (or areas) and the nature
of the electrolyte. The effect is more pronounced in immersed and buried objects. The
larger the cathode, the more aggressive the attack on the anode. Where the presence of
electrolyte is limited, the effect on mild steel sections is minimal and for most practical
building applications where moisture is controlled, no special precautions are needed. For
greater risk areas where moisture will be present, gaskets, bushes, sleeves or paint systems
can be used to separate the metal surfaces.
The galvanic series
Anode
Magnesium
Zinc
Aluminium
Carbon and low alloy steels (structural steel)
Cast iron
Lead
Tin
Copper, brass, bronze
Nickel (passive)
Titanium
Stainless steels (passive)
Cathode
Building Elements, Materials, Fixings and Fastenings 301
Structural adhesives
There is little definite guidance on the use of adhesives in structural applications which
can be considered if factory controlled conditions are available. Construction sites rarely
have the quality control which is required. Adhesive manufacturers should be consulted
to ensure that a suitable adhesive is selected and that it will have appropriate strength,
durability, fire resistance, effect on speed of fabrication, creep, surface preparation,
maintenance requirements, design life and cost. Data for specific products should be
obtained from manufacturers.
Adhesive families
Epoxy resins Good gap filling properties for wide joints, with good strength and durability;
low cure shrinkage and creep tendency and good operating temperature
range. The resins can be cold or hot cure, in liquid or in paste form but
generally available as two part formulations. Relatively high cost limits their
use to special applications.
Polyurethanes Very versatile, but slightly weaker than epoxies. Good durability properties
(resistance to water, oils and chemicals but generally not alkalis) with
operating temperatures of up to 60
·
C. Moisture is generally required as a
catalyst to curing, but moisture in the parent material can adversely affect the
adhesive. Applications include timber and stone, but concrete should
generally be avoided due to its alkalinity.
Acrylics Toughened acrylics are typically used for structural applications which
generally need little surface preparation of the parent material to enhance
bond. They can exhibit significant creep, especially at higher operating
temperatures and are best suited to tight fitting (thin) joints for metals and
plastics.
Polyesters Polyesters exhibit rapid strength gain (even in extremely low temperatures) and
are often used for resin anchor fixings etc. However they can exhibit high cure
shrinkage and creep, and have poor resistance to moisture.
Resorcinol-
formaldehydes (RF) and
phenol-resorcinol-
formaldehydes (PRF)
Intended for use primarily with timber. Curing can be achieved at room
temperature and above. These adhesives are expensive but strong, durable,
water and boil proof and will withstand exposure to salt water. They can be
used for internal and external applications, and are generally used in thin
layers, e.g. finger joints in glulam beams.
Phenol-formaldehydes
(PF)
Typically used in factory `hot press' fabrication of structural plywood. Cold
curing types use strong acids as catalysts which can cause staining of the
wood. The adhesives have similar properties to RF and PRF adhesives.
Melamine-urea-
formaldehydes (MUF)
and urea-formaldehydes
(UF)
Another adhesive typically used for timber, but these need protection from
moisture. These are best used in thin joints (of less than 0.1mm) and cure
above 10
·
C.
Caesins Derived from milk proteins, these adhesives are less water resistant than MUF
and UF adhesives and are susceptible to fungal attack.
Polyvinyl acetates and
elastomerics
Limited to non-loadbearing applications indoors as they have limited moisture
resistance.
Adhesive tapes Double sided adhesive tapes are typically contact adhesives and are suitable
for bonding smooth surfaces where rapid assembly is required. The tapes have
a good operating temperature range and can accommodate a significant
amount of strain. Adhesive tapes are typically used for metals and/or glass in
structural applications.
302 Structural Engineer's Pocket Book
Surface preparation of selected materials in glued joints
Surface preparation is essential for the long-term performance of a glued joint and the
following table describes the typical steps for different materials. Specific requirements
should normally be obtained from the manufacturer of the adhesive.
Material Surface preparation Typical adhesive
Concrete 1. Test parent material for integrity Epoxies are commonly used with
concrete, while polyesters are used in
resin fixings and anchors. Polyurethanes
are not suitable for general use due to
the alkalinity of the concrete
2. Grit blast or water jet to remove the
cement rich surface, curing agents and
shutter oil, etc.
3. Vacuum dust and clean surface with
solvent approved by the glue manufacturer
4. Apply a levelling layer to the roughened
concrete surface before priming for the
adhesive
Steel and cast iron 1. Degrease the surface Epoxies are the most common for use
with structural iron/steel. Where high
strength is not required acrylic or
polyurethane may be appropriate, but
only where humidity can be controlled or
creep effects will not be problematic
2. Mechanically wire brush, grit blast or water
jet to remove millscale and surface coatings
3. Vacuum dust then prime surface before
application of the adhesive
Zinc coated steel 1. Test the steel /zinc interface for integrity Epoxies are suitable for structural
applications. Acrylics are not generally
compatible with the zinc surface
2. Degrease the surface
3. Lightly abrade the surface and avoid
rupturing the zinc surface
4. Vacuumdust and then apply an etch primer
5. Thoroughly clean off the etch primer and
prime the surface for the adhesive
Stainless steel Factory method: Toughened epoxies are normally used for
structural applications 1. Acid etch the surface and clean thoroughly
2. Apply primer
Site method:
1. Degrease the surface with solvent
2. Grit blast
3. Apply chemical bonding agent, e.g. silane
Aluminium Factory method: Epoxies and acrylics are most commonly
used. Anodized components are very
difficult to bond
1. Degrease with solvent
2. Use alkaline cleaning solution
3. Acid etch, then neutralize
4. Prime surface before application of the
adhesive
Site method (as 1 and 2):
3. Grit blast
4. Apply a silane primer/bonding agent
Timber 1. Remove damaged parent material
2. Dry off contact surfaces and ensure both
surfaces have a similar moisture content
(which is also less than 20)
3. Plane to create a clean flat surface or lightly
abrade for sheet materials
4. Vacuum dust then apply adhesive promptly
Epoxies are normally limited to special
repairs. RF and PRF adhesives have long
been used with timber. Durability of the
adhesive must be carefully considered.
They are classified:
WBP±Weather Proof and Boil Proof
BR±Boil Resistant
MR±Moisture Resistant
INT±Interior
Plastic and fibre
composites
1. Dust and degrease surface Epoxies usual for normal applications. In
dry conditions polyurethanes can be
used, and acrylics if creep effects are not
critical
2. Abrade surface to remove loose fibres and
resin rich outer layers
3. Remove traces of solvent and dust
Glass 1. Degreasing should be the only surface
treatment. Abrading or etching the surface
will weaken the parent material
Structural bonding tape or modified
epoxies. The use of silicon sealant
adhesives if curing times are not critical
2. Silane primer is occasionally used
Building Elements, Materials, Fixings and Fastenings 303
Fixings and fastenings
Although there are a great number of fixings available, the engineer will generally specify
nails, screws or bolts. Within these categories there are variations depending on the materials
to be fixed. The fixings included here are standard gauges generally available in the UK.
Selected round wire nails to BS 1202
Length
mm
Diameter (standard wire gauge (swg) and mm)
11 swg 10 9 8 7 6 5
3.0 mm 3.35 3.65 4.0 4.5 5.0 5.6
50
. .
75
. . .
100
. . .
125
.
150
.
Selected wood screws to BS 1210
Length
mm
Diameter (standard gauge (sg) and mm)
6 sg 7 8 10 12 14 16
3.48 mm 3.50 4.17 4.88 5.59 6.30 6.94
25
. . . . .
50
. . . . . . .
75
. . . . . .
100
. . . .
125
. . . .
Selected self-tapping screws to BS 4174
Self-tapping screws can be used in metal or plastics, while thread cutting screws are
generally used in plastics or timber.
AB D U BF B
Metals +plastics Plastics Metals
304 Structural Engineer's Pocket Book
Selected ISO metric black bolts to BS 4190 and BS 3692
Bolt head
Shank
Pitch – the
distance between
points of threads
Thread
Nut Nut Dome
head
nut
Nominal
diameter
mm
Coarse
pitch
mm
Maximum
width of
head and
nut
mm
Maximum
height of
head
mm
Maximum
thickness
of nut
(black)
mm
Minimum
distance
between
centres
mm
Tensile
stress
area
mm
2
Normal size* (Form E)
round washers to
BS 4320
Across
flats
Across
corners
Inside
diameter
mm
Outside
diameter
mm
Nominal
thickness
mm
M6 1.00 10 11.5 4.375 5.375 15 20.1 6.6 12.5 1.6
M8 1.25 13 15.0 5.875 6.875 20 36.6 9.0 17.0 1.6
M10 1.50 17 19.6 7.450 8.450 25 58.0 11.0 21.0 2.0
M12 1.75 19 21.9 8.450 10.450 30 84.3 14.0 24.0 2.5
M16 2.00 24 27.7 10.450 13.550 40 157.0 18.0 30.0 3.0
M20 2.50 30 34.5 13.900 16.550 50 245.0 22.0 37.0 3.0
M24 3.00 36 41.6 15.900 19.650 60 353.0 26.0 44.0 4.0
M30 3.50 46 53.1 20.050 24.850 75 561.0 33.0 56.0 4.0
* Larger diameter washers as Form F and Form G are also available to BS 4320.
Length*
mm
Bolt size
M6 M8 M10 M12 M16 M20 M24
30
. .
50
. . . . .
70
. . . . . .
100
. . . . .
120
. . . . .
140
. . .
150
. . .
180
.
* Intermediate lengths are available.
M6, M8, M10 and M12 threaded bar (called studding) is also available in long lengths.
Spanner and podger dimensions
3.2 D
Spanner
2
(
D
+
1
)
Podger
15°
15°
Building Elements, Materials, Fixings and Fastenings 305
Selected metric machine screws to BS 4183
Available in M3 to M20, machine screws have the same dimensions as black bolts but
they are threaded full length and do not have a plain shank. Machine screws are often
used in place of bolts and have a variety of screw heads:
Pozidrive
Hexagonal
Phillips
Square Allen key Fillister
Pig nose Counter sunk
Cross slot Round
Slotted Cheese
Selected coach screws to BS 1210
Typically used in timber construction. The square head allows the screw to be tightened
by a spanner.
Length*
mm
Diameter
6.25 7.93 9.52 12.5
25
. .
37.5
. . .
50
. . .
75
. . . .
87.5
. .
100
. . . .
112
.
125
. . . .
150
. . .
200
.
Selected welding symbols to BS 449
Fillet Vee butt Double vee butt Spot
Seam Spot flush one side Spot flush two sides
306 Structural Engineer's Pocket Book
Cold weather working
Cold weather and frosts can badly affect wet trades such as masonry and concrete;
however, rain and snow may also have an effect on ground conditions, make access to
the site and scaffolds difficult, and cause newly excavated trenches to collapse. Site staff
should monitor weather forecasts to plan ahead for cold weather.
Concreting
Frost and rain can damage newly laid concrete which will not set or hydrate in tempera-
tures below 1
·
C. At lower temperatures, the water in the mixture will freeze, expand and
cause the concrete to break up. Heavy rain can dilute the top surface of a concrete slab
and can also cause it to crumble and break up.
.
Concrete should not be poured below an air temperature of 2
·
C or if the temperature is
due to fall in the next few hours. Local conditions, frost hollows or wind chill may
reduce temperatures further.
.
If work cannot be delayed, concrete should be delivered at a minimum temperature of
5
·
C and preferably at least 10
·
C, so that the concrete can be kept above 5
·
C during the
pour.
.
Concrete should not be poured in more than the lightest of rain or snow showers and
poured concrete should be protected if rain or snow is forecast. Formwork should
be left in place longer to allow for the slower gain in strength. Concrete which has
achieved 5 N/mm
2
is generally considered frost safe.
.
Mixers, handling plant, subgrade/shuttering, aggregates and materials should be free
from frost and be heated if necessary. If materials and plant are to be heated, the
mixing water should be heated to 60
·
C. The concrete should be poured quickly and in
extreme cases, the shuttering and concrete can be insulated or heated.
Bricklaying
Frost can easily attack brickwork as it is usually exposed on both sides and has little bulk to
retain heat. Mortar will not achieve the required strength in temperatures below 2
·
C.
Work exposed to temperatures below 2
·
C should be taken down and rebuilt. If work
must continue and a reduced mortar strength is acceptable, a mortar mix of 1 part
cement to 5 to 6 parts sand with an air entraining agent can be used. Accelerators are not
recommended and additives containing calcium chloride can hold moisture in the
masonry resulting in corrosion of any metalwork in the construction.
.
Bricks should not be laid at air temperatures below 2
·
C or if the temperature is due to
fall in the next few hours. Bricklaying should not be carried out in winds of force 6 or
above, and walls without adequate returns to prevent instability in high winds should
be propped.
.
Packs, working stacks and tops of working sections should be covered to avoid soaking,
which might lead to efflorescence and/or frost attack. An airspace between any poly-
thene and the brickwork will help to prevent condensation. Hessian and bubble wrap
can be used to insulate. The protection should remain in place for about 7 days after
the frost has passed. In heavy rain, scaffold boards nearest the brickwork can be turned
back to avoid splashing, which is difficult to clean off.
.
If bricks have not been dipped, a little extra water in the mortar mix will allow the bricks
to absorb excess moisture from the mortar and reduce the risk of expansion of the
mortar due to freezing.
Building Elements, Materials, Fixings and Fastenings 307
Effect of fire on construction materials
This section is a brief summary of the effect of fire on structural materials to permit a
quick assessment of how a fire may affect the overall strength and stability of a structure.
It is necessary to get an accurate history of the fire and an indication of the temperatures
achieved. If this is not available via the fire brigade, clues must be gathered from the site
on the basis of the amount of damage to the structure and finishes. At 150
·
C paint will
be burnt away, at 240
·
C wood will ignite, at 400±500
·
C PVC cable coverings will be
charred, zinc will melt and run off and aluminium will soften. At 600±800
·
C aluminium
will run off and glass will soften and melt. At 900±1000
·
C most metals will be melting
and above this, temperatures will be near the point where a metal fire might start.
The effect of heat on structure generally depends on the temperature, the rate and
duration of heating, and the rate of cooling. Rapid cooling by dousing with water
normally results in the cracking of most structural materials.
Reinforced concrete
Concrete is likely to blacken and spall, leaving the reinforcement exposed. The heat will
reduce the compressive strength and elastic modulus of the section, resulting in cracking
and creep/permanent deflections. For preliminary assessment, reinforced concrete heated
to 100±300
·
C will have about 85% of its original strength, by 300±500
·
C it will have
about 40% of its original strength and above 500
·
C it will have little strength left. As it is a
poor conductor of heat only the outer 30±50 mm will have been exposed to the highest
temperatures and therefore there will be temperature contours within the section which
may indicate that any loss of strength reduces towards the centre of the section. At about
300
·
C concrete will tend to turn pink and at about 450±500
·
C it will tend to become a
dirty yellow colour. Bond strengths can normally be assumed to be about 70% of pre-fire
values.
308 Structural Engineer's Pocket Book
Prestressed concrete
The concrete will be affected by fire as listed for reinforced concrete. More critical is the
behaviour of the steel tendons, as non-recoverable extension of the tendons will result in
loss of prestressing forces. For fires with temperatures of 350±400
·
C the tendons may
have about half of their original capacity.
Timber
Timber browns at 120±150
·
C, blackens at 200±250
·
C and will ignite and char at
temperatures about 400
·
C. Charring may not affect the whole section and there may
be sufficient section left intact which can be used in calculations of residual strength.
Charring can be removed by sandblasting or planing. Large timber sections have often
been found to perform better in fire than similarly sized steel or concrete sections.
Brickwork
Bricks are manufactured at temperatures above 1000
·
C, therefore they are only likely to
be superficially or aesthetically damaged by fire. It is the mortar which can lose its
strength as a result of high temperatures. Cementitious mortar will react very similarly
to reinforced concrete, except without the reinforcement and section mass, it is more
likely to be badly affected. Hollow blocks tend to suffer from internal cracking and
separation of internal webs from the main block faces.
Steelwork
The yield strength of steel at 20
·
C is reduced by about 50% at 550
·
C and at 1000
·
C it is
10% or less of its original value. Being a good conductor of heat, the steel will reach the
same temperature as the fire surrounding it and transfer the heat away from the area to
affect other remote areas of the structure. Steelwork heated up to about 600
·
C can
generally be reused if its hardness is checked. Cold worked steel members are more
affected by increased temperature. Connections should be checked for thread stripping
and general soundness. An approximate guide is that connections heated to 450
·
C will
retain full strength, to 600
·
C will retain about 80% of their strength and to 800
·
C will
retain only about 60% of their strength.
Building Elements, Materials, Fixings and Fastenings 309
Aluminium
Aluminium is extracted from ore and has little engineering use in its pure form. Alumi-
nium is normally alloyed with copper, magnesium, silicon, manganese, zinc, nickel and
chromium to dramatically improve its strength and work hardening properties.
Aluminium has a stiffness of about one third of that for steel and therefore it is much
more likely to buckle in compression than steel. The main advantage of aluminium is its
high strength:weight ratio, particularly in long span roof structures. The strength of cold
worked aluminium is reduced by the application of heat, and therefore jointing by bolts
and rivets is preferable to welding.
For structural purposes wrought aluminium alloy sections are commonly used. These are
shaped by mechanical working such as rolling, forging, drawing and extrusion. Heat
treatments are also used to improve the mechanical properties of the material. This
involves the heating of the alloy followed by rapid cooling, which begins a process of
ageing resulting in hardening of the material over a period of a few days following the
treatment. The hardening results in increased strength without significant loss of ductility.
Wrought alloys can be split into non-heat treatable and heat treatable according to the
amount of heat treatment and working received. The temper condition is a further
classification, which indicates the processes which the alloy has undergone to improve
its properties. Castings are formed from a slightly different family of aluminium alloys.
Summary of material properties
Density 27.1 kN/m
3
Poisson's ratio 0.32
Modulus of elasticity, E 70 kN/mm
2
Modulus of ridigity, G 23 kN/mm
2
Linear coefficient of
thermal expansion
24 ×10
÷6
/
·
C
Notation for the classification of structural alloys
Heat treatable alloys T4 Heat treated ± naturally aged
T6 Heat treated ± artificially aged
Non-heat treatable alloys F Fabricated
O Annealed
H Strain hardened
310 Structural Engineer's Pocket Book
Summary of main structural aluminium alloys to BS 8118
Values of limiting stresses depend on whether the products are extrusions, sheet, plate or drawn tubes.
Alloy Temper Types of
product*
Typical
thicknesses
mm
Durability Approx. loss
of strength
due to
welding (%)
Limiting stresses
P
y
N/mm
2
P
c
/ P
t
N/mm
2
P
v
N/mm
2
Heat
treatable
6063 T4 Thin walled extruded sections
and tubes as used in curtain
walling and window frames
1±150 B 0 65 85 40
T6 1±150 50 160 175 95
6082 T4 Solid and hollow extrusions 1±150 B 0 115 145 70
T6 1±20 50 255 275 155
20±150 50 270 290 160
Non-heat
treatable
5083 O Sheet and plate. Readily
welded. Often used for
plating and tanks
0.2±80 A 0 105 150 65
F
H22
3±25
0.2±6
0
45
130
235
170
270
75
140
LM 5 F Mainly sand castings
in simple shapes with
high surface polish
± A ± Strengths of castings determined in consultation
with castings manufacturer. Approx. values:
LM 6 F Good for
complex
shaped
castings
Sand
castings
Chill castings
±
±
B ± 40±120 70±140 25±75
*British Aluminium Extrusions do a range of sections in heat treatable aluminium alloys.
Source: BS 8118: Part 1: 1997
3
1
1
Durability
Corrosion protection guidelines are set out in BS 8118: Part 2. Each type of alloy is graded
as A or B. Corrosion protection is only required for A rated alloys in severe industrial,
urban or marine areas. Protection is required for B rated alloys for all applications where
the material thickness is less than 3mm, otherwise protection is only required in severe
industrial, urban or marine areas and where the material is immersed in fresh or salt
water.
Substances corrosive to aluminium include: timber preservatives; copper naphthanate,
copper-chrome-arsenic or borax-boric acid; oak, chestnut and western red cedar unless
they are well seasoned; certain cleaning agents and building insulation. Barrier sealants
(e.g. bituminous paint) are therefore often used.
Fire protection
Aluminium conducts heat four times as well as steel. Although this conductivity means
that `hot spots' are avoided, aluminium has a maximum working temperature of about
200 to 250
·
C (400
·
C for steel) and a melting temperature of about 600
·
C (1200
·
C for
steel). In theory fire protection could be achieved by using thicker coatings than those
provided for steel, aluminium is generally used in situations where fire protection is not
required. Possible fire protection systems might use ceramic fibre, intumescent paints or
sacrificial aluminium coatings.
Selected sizes of extruded aluminium sections to BS 1161
Section type Range of sizes (mm)
Minimum Maximum
Equal angles 30 ×30 ×2.5 120 ×120 ×10
Unequal angles 50 ×38 (web 3, flange 4) 140 ×105 (web 8.5, flange 11)
Channels 60 ×30 (web 5, flange 6) 240 ×100 (web 9, flange 13)
I sections 60 ×30 (web 4, flange 6) 160 ×80 (web 7, flange 11)
Tee sections 50 ×38 ×3 120 ×90 ×10
Rolled plates in thicknesses of 6.5±155 mm can be obtained in widths up to 3 m and
lengths up to 15 m.
312 Structural Engineer's Pocket Book
Structural design to BS 8118: Part 1
Partial safety factors for applied loads
BS 8118 operates a two tier partial safety factor system. Each load is first factored
according to the type of load and when loads are combined, their total is factored
according to the load combination. Dynamic effects are considered as imposed loads
and must be assessed to control vibration and fatigue. This is not covered in detail in BS
8118 which suggests `special' modelling.
Primary load factors
Secondary load factors for load combinations
Partial safety factors for materials depending on method of jointing
Comment on aluminium design to BS 8118
As with BS 5950 for steel, the design of the structural elements depends on the classifica-
tion of the cross section of the element. An initial estimate of bending strength would be
M
b
= p
y
S,g
m
but detailed reference must be given to the design method in the code.
Strength is usually limited by local or overall buckling of the section and deflections often
govern the design.
Source: BS 8118: Part 1: 1997.
Load type g
f1
Dead 1.20 or 0.80
Imposed 1.33
Wind 1.20
Temperature effects 1.00
Load combinations g
f2
Dead load 1.0
Imposed or wind load giving the most severe loading action on
the component
1.0
Imposed or wind load giving the second most severe loading
action on the component
0.8
Imposed or wind load giving the third most severe loading
action on the component
0.6
Imposed or wind load giving the fourth most severe loading
action on the component
0.4
Type of construction g
m
Members Joints
Riveted and bolted 1.2 1.2
Welded 1.2 1.3 or 1.6
Bonded/glued 1.2 3.0
Building Elements, Materials, Fixings and Fastenings 313
13
Useful Mathematics
Trigonometric relationships
Addition formulae
sin(A ±B) = sinAcos B ±cos AsinB
cos(A ±B) = cos Acos B (sinAsinB
tan(A ±B) =
tanA ±tanB
1 (tanAtanB
Sum and difference formulae
sin A ÷sin B = 2 sin
1
2
(A ÷B) cos
1
2
(A ÷B)
sin A ÷sin B = 2 cos
1
2
(A ÷B) sin
1
2
(A ÷B)
cos A ÷cos B = 2 cos
1
2
(A ÷B) cos
1
2
(A ÷B)
cos A ÷cos B = ÷2 sin
1
2
(A ÷B) sin
1
2
(A ÷B)
tan A ÷tan B =
sin(A ÷B)
cos Acos B
tan A ÷tan B =
sin(A ÷B)
cos Acos B
Product formulae
2 sin Acos B = sin(A ÷B) ÷sin(A ÷B)
2 sin AsinB = cos(A ÷B) ÷cos(A ÷B)
2 cos Acos B = cos(A ÷B) ÷cos(A ÷B)
Multiple angle and powers formulae
sin 2A = 2 sinAcos A
cos 2A = cos
2
A ÷sin
2
A
cos 2A = 2 cos
2
A ÷1
cos 2A = 1 ÷2 sin
2
A
tan 2A =
2 tan A
1 ÷tan
2
A
sin
2
A ÷cos
2
A = 1
sec
2
A = tan
2
A ÷1
Relationships for plane triangles
Pythagoras for right
angled triangles: a
2
÷b
2
= c
2
Sin rule:
a
sinA
=
b
sin B
=
c
sin C
sin A =
2
bc